First off, repeating fields aren't the best approach for this. To see an example of a better approach, create a copy of the Invoices starter solution that comes with filemaker 11. (Invoices and purchase orders are the same thing except for the direction the money travels during the transaction.)
Even so, you should be able to get a summary report with sub totals for each vendor from your purchase order table. I can't tell from your original post how you've set this up though.
How do you compute the total for each PO? Are those totals correct on each PO?
How are you computing the summary report subtotals? with a summary field?
What layout parts do you have in your summary report? How are they sorted?
1. Each PO total is computed with a field called "subtotal" which is a calculation field... sum(Purchase Price). These totals are all correct.
2. Summary Report subtotals are computed using a summary field called "total purchases" which is a summary of the "subtotal" field.
3. The summary report is in three parts:
Sub-summary sorted by "subtotal"
Trailing Grand Summary which contains the "total purchases" field
"sorted by subtotal" doesn't look right. That is what you would use if you wanted to group your PO's so that all POs of the same total value are in one group above or below the subsummary part.
From your first post, I'd guess you would sort by a vendor field so that PO's to the same vendor would be grouped together and your Sub summary would be used to to show the subtotal for that vendor. In which case, the "sorted by" field in the subsummary part would be a field that identifies the vendor.
This should be obvious to you, but the records will also need to be sorted in an order that includes the subsummary's "sorted by" field inorder for the sub summary part to be visible.