If your lists are clean you can create a new one as a SQL list and show Results from the Vendor Variable.
Create a New SQL List
Paste this sql code
Select distinct Vv.ValueCache
From VariableValue Vv
Where Vv.VariableID = ( select variableid
where Variablename like 'Vendor Name') And
Vv.ValueCache != ''
order by Vv.ValueCache ASC
Change the var to you needs.
Set user, pass and connection to the EPDM DB
The new list will list results already inside you DB
Another option would be to place a Read-Only editbox on your datacard that uses an Input Formula to combine the selections from Vendor A list, Vendor B list, and Vendor C list into a single field / variable, say 'CombinedVendor'.
Then, on your search card, search the 'CombinedVendor' variable, and if a given vendor was selected in A, B, or C, it would return results when searched. Also, if you add added values to any of those lists in the future, you would not need to make any other changes and the system would still work as desired.
Please let me know if you have questions or need additional help setting something like this up.