Yellow mark just means they want to talk, that's how you advance his personal side quest
tells you he his a vampire, gives you his background and when you run into the monster Hunter who's hunting astarion. It puts all the pieces together. His master(a vampire Lord) is sending out people to retrieve him and bring him back. Without talking to him, you'd have very little information besides that he looks like a vampire
I was talking about tieflings party. About him being vampire, he has night cutscene with feeding, not really yellow mark thingy. Like I said, I had him at recruitment time and have dumped him for Laezel fast. Beside yellow mark situations all I heard from him was "you again" ;-) People complain about companions being horny all at once. I have proposed easy fix, so we get our romances locked fast and don't have to deny unwanted attention from other NPCs.
