Vampires can see other vampires but only if they rejoin.

[Bologna Prime]

This is something I've experienced a few times and is a bug one way or another. Vampires can see if someone else is a vampire with a little red V over their head; I don't know if this is an intended feature but they can only see each other if they rejoin the game after they've spawned as a vampire. I'm not sure if it happens with serum vampires but you can test this by being selected as a vampire and logging out and back into the game and poof you and all other vampires have a little red V over their head. The bug is either it only happens upon rejoin or that it happens upon rejoin.