I done almost every main NCR missions, but then i decided to blow both the legion and NCR up ind the end of LR, and now im with Yes man. I'm ready to take the dam, but i can't trigger for auld lang syne. When i ask arcade, he thinks i'm still with NCR, what do i do?

Did you bring Arcade around the different triggers for points? Like the crashed Vertibird or the Van Graffs?

Not the van graff's, but will the veritibird alone be enough?

Nope, bring him to the Van Graff's shop, then REPCONN HQ. Then he should be ready, if you're far enough along in the main quest.

I [censored] the van graffs up - will it still work?

Yep, you just have to bring him around places with tech. I forgot the other areas but there a quite a few.