This is more of a MythTV question.. but I'll answer it here..
If you are watching LiveTV (most likely via Myth) and another MD (of myth client) attempts to use that tuner, Myth will tell the second client that all tuners are being used, and ask if it wants to 'watch' the current program being tuned. (That last part is in the SVN of myth, I don't know if it's in .20)
So, LiveTV clients are all on a first come first serve basis.
That being said, Recordings DO have a priority.. If you are watching a show, and Myth needs a tuner to record a different show, it will ask you what to do... (on screen)
(I'm not sure how VDR will handle this, I assume very similar)