You can do this by modifying the datacard so the variable PartID uses the filename without the extension. (see image)
When you add files to the vault, it will use the filename. Keep in mind that it only does this on the initial adding of the file to the vault. If you change the filename you will need to manually change the datacard.
However, if you prefer to do it for specific files after adding to the vault, you can create a Dispatch action file that allows you to select the files, read the filename and fill out the variable on the datacard. (see video example) - to learn more about dispatch, google 'SOLIDWORKS World Presentations' and search on presenter name 'Garon'. (I put the link to presentation below)
Thanks a lot for this, really appreciate your time. The example dispatch was exactly what I was after and with a quick tweak of the variable names I have it working! Really chuffed as I've been stuck on this for a couple of days now.