I recorded a video to show what happened. It's not the best Quality, but it does the job. Basically if you cancel the VehicleExitEvent to stop a player leaving a vehicle, it will cancel it once, but not any time after that. You cannot leave the vehicle, but the event is not fired, and the server thinks you're not in a vehicle.

Here's the video:
https://youtu.be/Q2tdrQQE8fM

Here's the code for the /checkvehicle plugin:
http://pastebin.com/F11uy5Aa

Output of /version:
[01:08:47] [Server thread/INFO]: This server is running CraftBukkit version git-Spigot-5391d73-287ef78 (MC: 1.10.2) (Implementing API version 1.10.2-R0.1-SNAPSHOT)
[01:08:47] [Server thread/INFO]: Checking version, please wait...
[01:08:48] [Thread-8/INFO]: You are running the latest version