I don't want my 19 year old girlfriend to go on a trip with me and my family to las vegas for my 21st birthday
Why wouldn't you want to be with your girlfriend on your 21st birthday?
Being with my family is like being kidnapped. I don't want my girlfriend to suffer that as well.
It is understandable to keep your partner away from an unpleasant experience. However, it is also true her being there will make the experience less crappy for you. Just ask her first before excluding her.
Nah, I want her to be far away from that, I need to hate every second of this, but she doesn't have to.
Don't you think you could at least explain the situation to her and let her decide for herself? She might be really upset if you just go on your birthday trip without her.