Where is the data stored so I can see who claimed what vehicles?

This plugin doesn't have a data file. It simply sets the OwnerID property of the vehicle entity. That property is saved across restarts, as dictated by vanilla game logic.

This means that there is currently no way to see who claimed a vehicle. It is possible to implement a plugin or feature to enumerate the current vehicles and see who their owners are, to essentially infer who claimed each vehicle, but this won't provide information about vehicles that have since been destroyed, and it won't tell you that a vehicle was unclaimed if you are allowing players to use that command.