Once I've decided to use a VIP, can I change my mind?

Sadly, no. At this time, there is no way to get him out of the elevator. I even put a VIP in the elevator (from the lobby), closed Tiny Death Star while he was still in there, re-opened tiny Death Star, and he was gone for good! [ :( ]

This is somewhat true. If the elevator is at level one and empty, the VIP climbs right in and you can't back out. If the elevator is moving or full, the VIP well get in when able and will present you with a red VIP button and the option to use or save.

This can be used to"cheat"the system. If you want to buy a random VIP but have several saved you can wait until someone is in the elevator "use" all your VIPs, with the lobby empty a random VIP well cost 2 bux instead of the higher price. Deliver your passenger and then save each VIP as they climb in.