I believe latency is part of it, so it must be adressed and be as low as possible - but I don't think it tells the whole story. Connecting feel with just latency is a mistake IMO.
Played for years with modeling. It bugged me, but I put up with it because it was a lot more convenient. Thing is he is Steve Vai. His threshold of where a big rig starts being inconvient is a lot higher then mine, starting with the fact that I have to carry my stuff myself. I doubt he had a full on modeling rig running, but I know he prefers real amps for monitoring.
When someone says "modeling feels weird" some take that as "it makes me unable to play". That's not it.
I wasn't implying he had a modeling rig, in fact I'm pretty positive Mr Nordegg (his tech) had an amp in the side stage.
My point was that this time he didn't depended on his rig right behind him and it's quite sure Whitesnake's stage was a silent one with amps being mic'd backstage or on iso booths, yet, Steve seemed pretty much on the spot for the gig, throwing out the "latency" out of the question. As DI pointed out, our brains adapt and I'm sure the musical muscle in Vai's brain can do it far better than ours.