vehicles without avrils = fail!  You could just as easily make a basement chamber with a tank in it that can't shoot at anything I think - doesn't matter if players can actually get to them.  As for flags I say put them in an impossible to get to location (like a cubby hole up on the ceiling) if they are required for the VCTF gametype  :D

Ooooh here's a crazy thought... put vehicles beyond glass walls so people can see them and wonder how the fuck to get to them.  If you go with tanks (I love tanks) outside the map you could have a short height but long width window for them to shoot through making their ability to kill almost none.  I don't want it to be a map where using vehicles is really a good way to get kills.