I have make things like this preprocesing the data
Can build a procedure than returns all the vendor names and addresses and then put this results at a memo field of a cursor with two fields: first field the parent key and the second field a memo to store the vendor list.
Then run a SQL query to join all the tables and the cursor.
After that on your report put a field for the memo and set stretch with overflow.
Thats all

Hope this help

Mauricio