Naritaverse

The Naritaverse is the unofficial name for the universe in which Vamp! is set alongside Baccano!, Durarara!!, and Etsusa Bridge, three other light novel series written by Ryohgo Narita that can be read in any order. The universe also includes Narita's novel Wednesdays Smiling with Otsuberu and his serial novel Pandora Shark.

Baccano!
Baccano! is the first and longest-running of Narita's original series, with twenty-two volumes and counting as of August 2018. Its overarching story spans 1700s-era Italy to 1930s-era America to the 2000s, involving the likes of alchemists, immortals, homunculi, mafiosi, assassins, and other curious characters whose lives all affect each other in significant and trivial fashion alike.

Durarara!!
Set in Tokyo's Ikebukuro district circa the early 2000s, Durarara!! depicts the lives of many characters and gangs of varyingly dangerous and eccentric personalities as they push and pull each other on the regular. These include: a dullahan courier; a manipulative information broker; the strongest man in Ikebukuro; the Russian Mafia; the internet-based group Dollars; the Yellow Scarves gang; and an underground doctor.

Aside from Baccano!, Durarara!! (illustrated by Suzuhito Yasuda) is the only other one of Narita's light novel series to have received an anime adaptation. The anime adapts all thirteen volumes of the main series, with Season 1 adapting Volumes 1-3, and Season 2 adapting Volumes 4-13. The sequel light novel series and the spin-off novels have not been animated.

Etsusa Bridge
Set in the late 2010s (2014; 2019-2021), Etsusa Bridge revolves around a gigantic, incomplete bridge connecting the Sado and Niigata islands and the nameless artificial floating city at its center. The city and bridge have become a haven for criminals, outcasts, and mafia outfits to make a dangerous life for themselves.

Wednesdays Smiling with Otsubure
Introduction forthcoming.

Pandora Shark
Introduction forthcoming.

Vamp! and Baccano!

 * In 1711 Whitesmile, Baccano! character Carla credits Carnald Strassburg as the engineer who designed House Dormentaire's floating barricade of ships stationed in Lotto Valentino's harbor. Lucrezia de Dormentaire remarks in the same novel that Carnald is also responsible for Growerth's perfect sewage facilities.
 * Several of Carnald's paintings hang in Nebula's headquarters in Chicago in 1934, which – along with Nebula's apparent fascination with him is remarked upon by Gustav St. Germain in the 1934 arc.
 * In Vamp! IV, Rude Gardastance muses over the Nebula "Mist Babel" headquarters in Chicago, as the skyscraper is visible from his penthouse. He mentions that he has a debt to Nebula's chairman from five generations ago - it is possible that this chairman is Karl Muybridge.
 * Rude is also referenced in 1934 Peter Pan in Chains, when Chairman Muybridge remarks that Ladd Russo should have gone to "Rude, Beriam, or one of those other fellows with money" if he had wanted to cause trouble for Nebula back when he was a youngster.
 * The Mars Clan sold the sailing vessel Advena Avis "at a great debt" to Dalton Strauss in 1711. It is unknown if Romy Mars personally conducted the transaction.
 * During the 1920s, Doubs Hewley belonged to a roaming American circus troupe which included the likes of Felix Walken, Gregoire, and Charkie before the troupe broke up circa 1925. Though Felix had alluded to a performer who claimed to be a vampire in the twentieth light novel, the vampire was only confirmed by name to be Doubs in 1935-D Luckstreet Boys
 * In 1710 Crack Flag, The Third Library is said to be funded by a "nobleman from an island in northern Prussia." It is more than likely that this island is actually Growerth, the main setting of Vamp!.
 * In 1935-B Dr. Feelgreed, Dalton recalls in 2003 that he has "seen werewolves and vampires on an island to the north." This island must be Growerth; when Dalton visited Growerth is unknown.
 * False Ending 21 of the Baccano! NDS Game essentially serves as an advertisement for Vamp!. It features Isaac and Miria aboard the Flying Pussyfoot, where they come across a moving mass of red liquid which forms itself into letters. Convinced that there is an invisible man vampire in the train, Isaac and Miria flee. The mass of blood is actually the vampire Gerhardt von Waldstein.

Anime

 * In Durarara!!x2 Ketsu Episode 1, Shingen mentions an "elderly video game-obsessed loli" and a "beautiful hungry-type werewolf girl" when he is discussing his work with Nebula to Walker Yumasaki. These are references to Romy and Watson respectively.

Light Novels

 * Vamp! and Durarara!! experienced a rare direct crossover with the short story Duramp!, which follows a succubus who arrives in Ikebukuro intending to feed off its residents' desires until she has enough power to take control of Growerth. She encounters several Durarara! characters over the course of the story, including Walker, Celty, Mikado Ryuugamine, Mika Harima, Seiji Yagiri, and Shizuo Heiwajima.
 * In the gaiden "The Two Getting Along," a black hole-like being is said to be visible in a photo of Waldstein Castle. This is a reference to Hawking.
 * One of Traugott Giessendörfer's various dojos is located in Ikebukuro, where Mikage Sharaku and Mairu Orihara learned his fighting style before moving to Rakuei Gym.
 * Rakuei Gym advertises itself as "Traugott Geissendorfer's Rakuei Gym," as the gym teaches one of Traugott's fighting styles. A poster of Traugott hangs outside the front entrance as per Durarara!! Volume 7.
 * The poster as featured in Episode 17 of Durarara!! x2 appears to be an advertisement for something called "Kenkonitteki the Final," promoting Traugott and other men as the "Ultimate Fighting Spirits." These other men include Eijirou Sharaku, Ken Aoyama, Dejan Bule, Rikimaru, and Andranita Haghighi.
 * Rakuei Gym appears to be using his name more as a marketing gimmick rather than out of any official affiliation, but Traugott evidently does not mind.
 * Shizuo became a fan of Traugott after Traugott was invited to speak at Raijin High School's closing ceremony during Shizuo's graduation year. He cites Traugott as his role model in Durarara!!x2 Episode 16, and the Raijin incident itself is adapted by Durarara!!x2 Ten Episode 1.5.
 * In Durarara!! Volume 3, Shingen cannot recall the answer to the crossword clue, "German Artist from [Growerth] Island." The answer would have been Carnald Strassburg.
 * A saleswoman attempts to sell a fake lithograph of one of Carnald's paintings to Chikage Rokujou in In Durarara!! Volume 5.
 * According to Kasane Kujiragi, the design of the demonic hammer Bannajin is inspired by—and a replica of—another hammer created by Carnald.
 * A discussion about vampire clans in Vamp! notes that the small Hijiribe Clan naturally dissipated due to assimilating into human society. Durarara!! character Ruri Hijiribe is a dhampyr with one-fourth vampiric heritage, as her grandmother was a vampire of said clan. As Kasane is her half-aunt on her mother's side, Kasane is also a dhampyr.
 * Durarara!! character Shinichi Tsukumoya is implied to be like QAWSED, a type of vampire which is supposedly fused with the digital plane.
 * In the character encyclopedia of Vamp! V, QAWSED says that "somethin' similar" to him is preventing him from taking control of an online network in Tokyo. A passage in Vamp! IV also writes, "It was rumored that [QAWSED] was in a conflict with a similar being somewhere in Japan over the digital plane..."
 * According to one of Narita's tweets, Masashi Mamiya and Manami Mamiya of Durarara!! are somehow related.
 * Chapter 1 of A Sunset with Izaya Orihara mentions that Hagane City, a city adjacent to Bunokara, is 'spacious enough' to serve as a sister city to German tourist cities. Hagane City is sister cities with Rukram, a city on Growerth that is now a part of Neuberg. According to Vamp! V, Masashi used to live in Hagane City before he left Japan for Germany.
 * In the third Durarara!! volume, Izaya tells Namie Yagiri to send a blue envelope to one Yamada of Hagane City. An Organization officer known as "Yamada the Pearl" has been mentioned more than once in the Vamp! novels; however, it has not been confirmed whether these Yamadas are the same person or are otherwise related.
 * Chapter 4 of the same novel (Sunset) reveals that one of the strongest martial artists in the Kanto region of Japan, Densuke Sozoro, taught Traugott the fundamentals of martial arts.
 * Rude Gardastance is Byakuyamaru Natsugawara's role model, according to the first interlude of Durarara!! SH Volume 4.
 * Rude is also referenced in Durarara!! Volume 11, where Shingen tweets that the "previous chairman of Nebula's business rival, the Gardastance Group, was a vampire."
 * In the first chapter of Durarara!! SH Volume 4, Celty recalls meeting a snake woman called Meline (メリーヌ) in one of France's forests and wonders how Meline is doing. This Meline is presumably the scylla Melina (メリーナ) from Vamp!, with the 'Meline' spelling likely one of Narita's typo. While Vamp! describes Melina as a scylla (スキュラ), SH 4 uses the kanji 蛇女 (furigana メリサンド Melisande, lit. 'snake woman') – defining Melina as Melusine/Melisande. This may be a mistake on Narita's part, as Melusine/Melisande is a folkloric figure rather than a species.

Vamp! and Etsusa Bridge

 * Etsusa Bridge character Gitarin shares his name with the vampire Laetitia Gitarin Aztanduja. As Laetitia has a habit of adopting children around the world, and 'Gitarin' is the one thing Gitarin never changes about his name, it is possible that Gitarin is either one of Laetitia's adopted children or otherwise connected to her.

Vamp! and Wednesdays Smiling with Otsuberu

 * An occult article in Otsuberu's Last Week Weekly tabloid mentions someone who might be either Rude or Watt Stalf.