I think Adam is right. It feels like he is testing you. Why pick Vegas as the destination at all if it makes him so uncomfortable? You work hard and should be able to do whatever you want with your money, no justifications necessary. And you shouldn't have to give up something you love (regardless of what it is) to keep him happy.
I don't want to overstep here but you mentioned that he's your first serious bf in many many years. I hope that you don't feel like you have to settle for this guy.
All in all, I hope that you can work it out before the trip. Would be a shame to waste time arguing or feeling uncomfortable on your trip, especially since you worked hard all year and this is somewhat of a reward.