It seems to be random. If you want you can use the unlockoutfits cheat to get Fangs, they are an Accessory.
I don't really want to use that cheat, because I'd rather unlock things while playing...
I wonder HOW the fangs "disappear" from the original townies? I'm sure when they created Beau Merrick, Vladimir Schlick and others, they gave them the fangs along with outfits and appearances. Why they all lost them in my game? Why just the fangs, if it's an accessory among others?
I met two new vampires (sorry, can't remember the names, but I assume they are original Bridgeport townies because I play with ageing off and nobody has moved in or out of town). They don't have fangs. My sim turned her BF into a vampire... and he does not have fangs.
In my game it seems to be 100%, not random. I'll try and make a new game in Bridgeport to try it out.
If you wonder why I care about such a tiny thing, it's just that I think vampires
should have pointy teeth. Now thay are just people with glowing eyes.