This isn't much of a speculation topic, but a compilation for those interested in what the Vampire Kids would've been like in the game. Most of what I state here are from the game's data.
In the trailers they were shown fighting in a cemetery. According to a map created by Cartman in the VGA 2012 trailer, the kids called this the "Land of the Dead". The cemetery would, presumingly, lead up to Mephisto's Lab, refered to as "Monster Lab". Both the Land of the Dead and Monster Lab are referenced in the game's code. The Cemetery is labeled as "e3 cemetery", and the Monster Lab is called in the strings "Mephisto's Lab". So far, I haven't been abled to load either, as Mephisto's Lab seemingly doesn't exist anymore in the files, unlike the cemetery. Another interesting thing to note is that there are also references to a "tomb lab" in the cemetery.
Mike Makowski, the leader of the Vampires, is in the data as well. His files have names similar to other important NPCs, hinting at a longer sidequest involving the Vampire Kids, or some kind of alliance or mini boss. There also are strings to Mike Makowski's House, and a "Sparkle Room". Strings also exist for close captioning if you were to fart on him, in which he says "P-P-Pee you!"
The Vampire Kids themselves were generic enemies, similar to how the Elves acted in the beginning of the game. They dropped Plastic Vampire Teeth, Grandpa Vampire Wigs, Hot Topic Catalogs, glitter, and "Vampire Blood", which was actually Clamato juice. Another file called Clammato juice is listed as a weapon in it's name, and appears to be a molotov ****tail, and appears similar to the ones the boys threw in the show. There were multiple types of the Vampires as well, referencing how Mike and the Vampire Kids had discussed about what kind of vampire they were in the episode "The Ungroundable". The game featured 2 of the Vampire types from the show, along with 3 new ones, countb, spar, and chal. The 2 types of Vampires originally from the show were psi and sanguinarian. The Countb costume can be seen here, modded onto Kyle. Presumingly, each vampire would have a different ability. At the moment, the found abilities are Immortal Blood, Levitatium Clottus, Vampiric Regeneration, and Full Moon.
Multiple items could be important to the plot, including a weapon called the Dawn Mower, and a book named "Dmeister", which appears similar to the book of lore Butters reads in the Ungroundable. The Dawn Mower was a scythe that appeared in some screenshots that showed the player fighting the vampire kids. The Dmeister was a quest item, that was also refered to as "vamp_book", in which the player would take it. Another item called the "Chalice" was used as a weapon, likely to hold the Clamato juice. It's possible the Chalice could be related to the "Chal" Vampire kid. The last item is the Altar, which could possibly be inside the Monster Lab with the Chalice as well.
Finally, 2 little quick notes; Henrietta, the female Goth kid, had a house that could be entered as well, and Butters was shown fighting the Vampire Kids in the posters released before THQ went bankrupt. This is important because all the Wallpapers before they were edited had shown the protaginists fighting enemies they had some kind of relation to, except for Kenny. Cartman was shown fighting the Aliens, who had placed the Anal Probe inside of him. Kyle was shown fighting the Crab People, who was one of the only people to find the secret about the Crab People, Stan was shown fighting the Woodland Critters, which he created in Imaginationland, and Jimmy was shown fighting the Goblins, a faction cut so early in development, that the references to them are from the VGA 2012 Trailer, and Jimmy's wallpaper. Their appearance on the poster means that most likely Jimmy would have some kind of connection to the scrapped faction, and they could've possibly set their base in Stark's Pond.
Take it all for granted, because it's just from the Data itselves, but if you want to look for yourself, you can open up the Stick of Truth's data with this page.