Answer

Generally, you can delete vehicles across different task fields to reduce the number of used vehicles.
In order to be able to delete a vehicle, you should terminate the vehicle via "Valid to" date.

 

This means that the vehicle can no longer be used in the trip planning after that "Valid to" date.
Depending on the data reduction settings (delete data records older than X days), the vehicle can be deleted at the "Valid to" date + X days, since there are no longer any dependencies, such as tours.