I'm watching that last Vai video now (Now We Run) and I think that's just how he naturally looks when he's soloing. Everyone is different in that respect, some people barely move a muscle in their faces, some look like their having a bowel movement etc. A very famous guitarist here in Sweden, Janne Schaffer, always has his tongue sticking out from the corner of his mouth. Apparently he tried to keep it in once, but then his playing collapsed...
Vai is a US frontman and entertainer, so I'd be far more surprised if he didn't do a lot of stuff on stage even though they don't contribute much the the actual sound he's making.
On a side note... A couple of years ago when I was in the student theatre band, one of saxophone girls came up with a theory after watching the lead gutiarist do a solo: That a player's solo face is the same as their orgasm face. We had a lot of fun at practice after that! :icon_jokercolor: