Is it possible (and legal) to play one game at the same time as you post you are on vacation in another

When you enable vacation, all your online games go on vacation at the same time.  When you come off vacation, they are all off vacation at the same time.

but if you are on vacation in a tournament that does not allow vacations, could you get in and out of vacation to play those games  in time ? I ask because some of my opponents are on vacation and in such a tournament at the same time!

Sorry, I should have added that there is that exception.  On-going games that were specified as being "no-vacation" games will continue going even after all other games are paused for vacation, and so it's still possible to time out in the no-vacation games if moves aren't made in time.  You don't have to come off vacation to make the moves in those no-vacation games, they'll just continue as if you hadn't gone on vacation at all.