We've seen vampires do magic spells. As far as the level of skill that a vampire might possess, I don't see any reason why just being a vampire would be any kind of limitation. (I'm not sure if Angelus' attempt to destroy the world in Season 2 counts or not, but that was right up there with Dark Willow in Season 6) We've seen other types of demons who can apparently control an apparently higher level of magic that Willow can, in fact, such as the Cave Dweller who restored Spike's soul. Certainly, Willow restored Angel's soul, not once, but twice, but she was only reinstating a previously existing spell, cast, supposedly, by some Gypsy witch, wizard, or sorcerer. The Cave Dweller actually created his spell which, I would think, would be much harder.