Well, technically Evil gods use UNHOLY symbols and Vampire Weaknesses specifically says HOLY symbols (and also, Vamp Clerics should still be allowed to use Unholy symbols without problem), but I think one of Orcus is appropriate. Either way, it's a cool idea. In fact, Froogleyboy, you should definitely check out the Weakness section of the Vampire entry and throw all KINDS of stuff from there at your pesky vamp character.