I'm resurrecting this topic to share useful information.
I had a firefighter with a Motive Mobile, and it seemed like no matter what I did, once she got the fire engine, she used that instead of the Motive Mobile, even if the fire engine was in family inventory! The thought occurred to me that if I traveled and got a treasure chest, maybe I could put the fire engine in the chest - but just traveling was enough to do the trick. Even though both vehicles ended up in my sim's inventory when she got to France (because she drove the fire engine to the airport, of course), she started using the Motive Mobile (which I had set as her preferred vehicle) to drive around France, and kept using it once she got back.
This makes me wonder if just switching households and switching back immediately would have been enough to do the trick, since traveling isn't always an option.
ETA: And she switched back to the fire truck after the first fire she responded to. Boo. Never mind, I guess.