Yes, you can delete an old vehicle from AAA AutoManager.  However, you must have a new vehicle created before you will be able to delete the old vehicle.  To delete the old vehicle, please make sure the correct vehicle is showing and then click on Edit next to the vehicle name.   Once you are on the Update Vehicle Information, at the bottom, you should see a Delete this Vehicle link.  Please click it and it will remove the old vehicle.  You will be asked to make sure you want to delete and just click on Delete.  You are then left with the one vehicle that is your current vehicle.
To go directly to Auto Manager, click here.