Should be the same thing, where are you seeing the fields?  

When in Sage 50's "Maintain Inventory Items" window under the general tab

and perhaps we created a specific record field in the Custom Fields tab for "Primary Vendor".

I want to make my data mining as effective as possible when I need the info at my finger tips.

Thank you for your quick response!

The Primary Vendor field is user defined so it woudl have been something that you or another user labeled within Maintain/Default Info/Inventory settings.  That field is available in a number of the report filters so it might be useful from that but the system uses the Preferred Vendor field when auto creating PO's so that is the one you would want your main vendors to be assigned.  You can always change the vendor on the PO if the system auto creates it.  

You can change the name of your "Primary Vendor" to say "Preferred Vendor" if that is better for you. The difference would be that you can type out the full vendor name in the custom field (or anything you want to add), but the Preferred Vendor field will have the vendor's assigned ID.

Options --> Default Info --> Inventory Items --> Custom Fields tab

Without knowing how you are using the two fields separately, it could be redundant and the custom fields could be used for something else you may need.