I thought that Vocal Adrenaline was supposed to be at regionals. I missed some episodes so maybe this was explained, but can anybody tell me why they weren't there?

I don't think they were supposed to be at the competition this year, and it is never explained why they weren't. They should have been there because they are both in the same region and we know that VA is continuing after Shelby left. As far as we know, there is not a limit on how many times you can win.

I believe they will competing at Nationsals

I think the reason is because The Warblers also won Sectionals so they had to bump VA into another regionals. But I'm not entirely sure. New Directions will be up against VA though at Nationals.