Psyringe: The game is described as a "2 on 2 fighting game". Does it have a single player mode? How are matches managed - does the game rely on the developer's server to be present? These are things that could influence GOG's decision whether to add it to the catalog.
The game is primarily single player with a story mode, it was programmed by one guy, supposedly an ex-capcom staffer. There isn't any online play without external software.
singbird: According to
http://wiki.mizuumi.net/w/Vanguard_Princess it is a 2D fighting game. I am actually not very surprised it got turned down, since GOG doesn't really have too many of these as I understand. I don't remember encountering a single one, although I don't browse the action category often. But probably they will accept it if enough people want it, if the "issue" is on GOG's side.
hedwards: There's a small number of them, Street Fighter 2 and some others. But, the PC has never been particularly known for this particular genre of game anyways.
This is precisely why this is NEEDED on GOG, go past the Anime meets Dead or Alive exterior and you'll find an extremely fluid and fun fighting engine, one of the best on the PC.