I think Tristan threw up because he was fighting the hypnotic spell that Ethan put on him... We can see that his eye color changed when Ethan transformed, so from then Tristan was under Ethan's spell... He was fighting to recover his senses and I think that's why he threw up...
Hmm yea that could be it
I don't think it's only because of him fighting the hypnosis... But because he realized that the guy he likes is the species he hunts? Like you know... He is disgusted by vampires and his so called "lover" ended up being one and he also happens to be his next kill assignment
They were furiously fucking and banging and then.... All of a sudden Tristan throw up?? But.... Why??? And the last but not least.... Tristan needs to choose to LOVE ETHAN!!!! No matter what!!!!!