Hm... if you wanna fight in the vehicle... I think this is a good way to do it... (lol doing this from memory and guessing...)
Have a tank event (for example) and on action key, have the character walk into and and remove the original character from the party and add the tank character then turn on switch Tank ON.
New page: blank charset, condition of Tank ON.
Then on a common event, have it like...
Conditional branch: when Key is pressed, remove tank, add original from party and turning off switch Tank ON.
Um... hopefully that'll work XD
Tell me if it works out. If not, then I'll throw something together quickly.