So I've enabled the mutator in Onslaught, but I don't see any changes to the vehicles, and nothing is different about the shooting. Is all there is to it modified damage?

The guns should fire completely differently with Ballistic Vehicles enabled. The hellbender gets a gatling gun and the Scorpion gets a rocket launcher if I remember correctly.

Oh, and if you choose the Ballistic Onslaught gametype, do you have to enable the Ballistic Vehicles mutator?

Yes, have to enable the mutator, too.

There's a few topics about this, one of my own in addition. I still have the issue, my end result is I just don't play with vehicles anymore. There was no fix found, and hasn't been one for a while. : /