[@Dethran]

Well, for full-blooded Vampires, all religious symbols will have an adverse effect of them. For example, Dracula (arguably one of the most powerful Vampires to exist in the Marvel universe) was negatively affected by a Star of David, but as he was both powerful and didn't believe in the Jewish faith at the time of his death (Being a Catholic, I believe - or, at the very least, Christian) he was able to get it away from him. Younger, weaker vampires will be more affected than that, but even more strongly so when it's a symbol that means something to them. Roran's immunity came from his pseudo-ness, not anything else.

So, even if you did run into an Atheist vampire, a symbol would likely effect them enough for you to have some slight advantage in getting away, at least.