Its not part of the game for most vehicles.
So probably not tourney legal in most cases.
I'm also generaly against putting bases on vehicles unless it'd be necessary to make them stand on the table - like flight stands. Or assorted walkers, Tau suits, etc. I've never yet seen a Crisis/broadside able to stand on its own.
So none of my
GSC vehicles or Ork buggies will ever have a base - if a Rhino doesn't need a base? Then I see no reason my Shok Dragsta would.
The two exceptions here are my Triarch Stalkers & my Soul Grinder.
I put them on 100mm(?) Rounds ages ago - to solve arguments from several opponents as to where b2b was.
The downside is that they can no longer be posed on/in terrain looking cool.
I also don't play tournaments - so the fact that my
GSC & Ork car aren't on bases isn't really a concern for anyone I play with.