One of my players has always complained that he wants to play a vampire. Honestly, its because of the stats, but that's what he cares about really. This player wants to be the very best like no one ever was and I somewhat get where he's coming from but I'd like the player power to be reasonably close.
If I give the other players Mythic powers (through story reasons) would it be fair to have him contract Vampirism or does that unbalance player power too much? Are there alternatives?
How many mythic levels will you give other players to compensate for the vampire template?
Also, don't let the player cheese his way out of the sunlight issue. Vampirism does have a rather distinct downside.
The running water thing is probably the most complicated to deal with.
I suppose Id have the other PCs grab mythic early and be able to develop it down the line to MR 4 to be even with vampirism. Until they reach MR 3 I'll have them be one level up on the Vampire PC. That fair?
He would be prone to cheeses, licensed rules lawyer. He would look through every book to find a loophole.
It depends on how optimized your other players are. Mythic is pretty cool, but it varies in "power" depending on how you use it. Talk to your players and see what they think is fair.