Auto add Images to entry after ftp to server
Since it sounds like you are using a third-party plugin for the initial import, you'll need a new plugin to handle the ftp uploads.
This new plugin would need to perform the following:


Basic control panel page, or Controller action for firing off the processing
Get the list of new images files from the FTP directory
For each file:


Parse the file name to get the associated vehicle stocknumber 
Query your vehicle entries based on the stock
Use the Assets Service  (https://craftcms.com/classreference/services/AssetsService) to
create and store the asset appropriately
Save the associated asset to the Vehicle Entry

At the end of the run, archive/remove the original uploaded files
Email the list of stocknumbers you have image files for but not existing entries (using EmailService)


To further optimize the process/queries you could query all vehicles first and pull them out of an array by the stocknumber (instead of querying the DB each file).

If you wanted to automate the process you could even build a task-listener to fire off your image-association plugin when it sees new images have been FTP'd to the vehicle image upload directory.