Hi Gary,
this can be achived by using sensitive fields for vendor, Please find the below screens for configuration, and this could be simplest way instead of validation.
Like wise you can maintain as many fields as you want. after this config you have to give authorization for T code FK08 who ever approving the changes or for new creation of vendors.
I hope this will resolve your issue, I have had the same requirement previously and implemented the same.
Regards,
Sree