Dawn,

It would be nice if the Finder and Smart Finder worked in the vendor transactions grid, however they do not. (Hint to R & D !!)

What you can do, although it is less handy, is:

Select Reports / Transaction Reports / Vendor transactions.
Enter the vendor number for start and end of the range and click Preview.
Enter the invoice number in the search box at the top and click on the binoculars. (You can also search for amounts this way.)