Hi Susan
That report uses the primary vendor at the PO/Receipt level. It ignores the vendor assigned to the detail line.
So what you want to do is use GridView and select the PO Header and/or Totals tables (I'm not sure which without doing more research) and create a report that mimics the Outstanding Receipt Invoice List, and summarize by Vendor. Then create a basic GridView report from Payables (the Vendor master table) that lists the vendor number, name and balance outstanding. Then bring the total from the first report onto the second one.
This is an advanced use of GridView and I can't go into all of the details here. If you get stuck, it's what we consultants do for a living.
Steve