To delete a vehicle in DispatchBot, simple go to More then Vehicles.

From the Vehicle list, click on the name of the vehicle you wish to Archive

In the bottom right corner of the Vehicle details page, click on the "Archive" button.

You will then be prompted on what to do with any trips that were assigned to the vehicle that you are archiving.  You can choose to re-assign them to a new vehicle or completely un-schedule them.  Click on "Save and archive vehicle" to finalize the archive process.