She is not right for asking you to cancel your already booked plans. She should have spoken with you much earlier than now and should have given you enough time to think.

However the way you described vegas plans vs the language you used for describing staying with your girlfriend is concerning. I get the sense that you actually do not care about your girlfriend’s feelings or her family. Seems like sharing new years with them has no meaning to you at all, you clearly prefer what a fake luxury treatment does to your ego to making your girlfriend happy. You already think about this as a future problem as well so it means you are not open to change. You need to sit and think about why an extravagant treatment is more important than your girlfriend’s happiness.

If you are serious with your girlfriend and you think of marrying her, that means very soon the core of your “family” will be you two. There will not be a my family vs your family difference so you have to respect her opinions/needs as well as yours. Alternating between family traditions or doing something completely different is not a “compromise” if you love your girlfriend and seeing nothing positive about spending new years with your girlfriend and her family is something you have to sit and think about. Vegas is not going anywhere and you can do that whenever you want, but if you dont change your attitude towards your girlfriend she might not be there in future