I primarily use one vehicle, but just took a road trip with the motorcycle so I switched primary vehicle while on the trip. Worked great. When I was filling up in a little town with no cell service (there were a few) I just saved the message as a draft and before I filled up the next time I sent it first.
Takes 2 seconds to switch primary vehicle, so this is a great work-around for me. It is MUCH easier (for me anyway) than using the mobile web interface.