Vehicles

The only gametypes that have vehicles are Assault, Onslaught and Vehicle CTF.

[These gametypes set bAllowVehicles=True]

Right click and add an ONS vehicle type. I am adding a tank:

 

Be sure to place the 2 FlagBases to spawn the vehicle.

Test level:

Name your map VCTF-Whatever ... iow, start with VCTF- for it to show up in the list of available game types.

Note -- by default, flying vehicles cannot carry a flag.
Another note -- The tank kills 3-4 players at once.