If Cassidy has spoken to your pup and told them that he's a vampire but doesn't have fangs, he lied. This is what happens when you play someone from ongoing canon! Details get changed along the way. And it's annoying! But at least lying about it is still within character. My headcanon now is that he casually lied about it because he doesn't want to be seen as a stereotypical vampire, and possibly didn't want to scare anyone away or freak anyone out. So he'll continue to lie about it. I think his fangs only come out with true bloodlust or the desire to turn someone, neither of which he really feels at this point in canon. He's learned to suppress those urges.