Being Vain? LAME!! Who said that that was a sign of being gay or bi? Whoever it was is ignorant.
The signs are noticable, if not from early childhood than at least from puberty. You find yourself physically and emotionally drawn to the same sex or both sexes. It's not a guessing game, if you walk like a duck, talk like a duck, have the feelings a duck would have for another duck, pop wood for a duck... then you is GAY.
About the bi-sexual guy being faithful: I would imagine that before the relationship gets too far at some point a conversation must be had. This conversation should include your sexuality, how you desire a person of the same sex while also have feelings for the opposite. Negotiations should begin, compromises should be reached so that if/when the time comes to live out your bi-sexual nature then it won't be a matter of faithfullness or not, it will be the natural course of things.
Honestly, it must be nice being a bi guy. I'm a totally gay guy, but I imagine that a bi-guy with a girlfriend/boyfriend has a lot more options. They can have three-ways in either direction if the female/male(s) are willing. You can explore the full spectrum of both sexes and have the desire to do so.