I have never until just now even known about that feature, that is interesting.
You should make a copy of your app just to be 100% sure you know the steps and that they also work. Test this first.
I think what you would need to do is go to your old table(lets call this Table A) that contained the Vendor field and create a report that contains all Vendor - Address records. I assume Table A now has the Vendor field as a lookup, right? So your report needs to have 2 columns, the Vendor field and the Vendor - Address field. Save that as a spreadsheet.
You will need to make a new Vendor - Address field on the new Vendor table as well. It might create it automatically with the import we do next but I like to do it myself.
Then you would go to your new Vendor table and import into it from csv and upload the spreadsheet we downloaded before. Set your merge field as the Vendor field and then import all of the Vendor - Address data into the newly created Vendor - Address field.
You need to then verify all your data is correct, or at least spot check it. Make sure the right addresses line up with the right Vendors and all that.
After this you would need to go back to Table A and delete the Vendor - Address field and then go to the relationship that was created between Vendor and Table A and create a lookup from Vendor back to Table A of the Vendor - Address field.
I think I got that all there but let me know if in your testing you find I may have missed something. Maybe there is even an easier way but I prefer to do it manually. Sometimes the "quick base magic" can burn you really hard.
