Yes, you got it right. And there's nothing wrong about that.
If Kataklysm weren't on the bill, I wouldn't go. But since they are, I might try to catch part of Vader's set. I hear they're good live.
Yes, definitely catch Vader. On their recent headlining tour they put on an INCREDIBLE show, played lots of crowd-pleasers. Their last song was a cover of Reigning Blood with Brett from Malevolent Creation on lead vocals. They'll probably only do that for another headlining tour, but I figure it's worth mentioning.