There is information on it, I believe, in both the Planewalker's Handbook, The Blood War, and other Planescape books... anything that discusses Gehenna or The Grey Wastes. I also seem to recall a single adventure in OP1: Tales of the Outer Planes that has an adventure in there featuring yugoloths, the main creature type of the plane.
Oh, and the Vampire thing has little to do with the AD&D thing.
Those who can, don't. Those who should not, do.