Make sure to run a trace first to determine if any special methods are firing off when that field gets updated. In PartPlant, the field you want is VendorNum. I believe in most places you update the VendorNum, not the VendorID. Be sure to trace a manual change of those fields before you set yourself on just an updatable BAQ. IT will probably work, but there could always be something hiding in the background that a trace will pick up.
Trace Helper Utility for Epicor ERP 10 - ERP 10 - Epicor User Help Forum (epiusers.help)
Good luck!
Edit: Make sure to close the trace file by unchecking trace and clicking OK.