You have "vehicle crews" and "infantry crews". If a vehicle crew mans / remans a vehicle its treated like a vehicle crew. (no penalties).
Yes, the guys can re-enter the vehicle in a subsequent turn. They can get into a different vehicle if you've been so unluckly as to have more than one unmanned vehicle...
By the way, this kind of question should be posted in the "Rules" section. This section is for (un)official answers to questions that MMP has answered.