Not sure if I understand.
You want a file called "Form" and in this file you want each vendors workbook.
The workbooks will be named with the "Vendors" name. Now each vendor will have more than 1 Invoice. If thats the case you fill in data for 1 invoice. What do want to do with the other invoices for that vendor.
I'm off to bed. But look at this tomorrow.
Let me know if my assumption is correct and add anymore ino that you can.