So I want to make an enchantment, a cast on strike one, with conditions attached to it, where ONLY vampire players benefit from it. I already know how to make sure it can't effect dwemer machines or something else equally odd on the receiving end, but how do I make this enchantment useless to non-vampire players?

I'm not really knowledgeable in these kinds of things, but could you take a look at Harkon's sword as a model? I'm fairly certain that has an enchantment that only works if the player is a vampire.