I just saw this older post so you may have already corrected this transaciton.

It depends on what you want to fix.  Just the invoice or the receipt of goods in inventory and GL also.

I would suggest doing return of goods on that PO, then fix the PO to the correct vendor and receive the invoice and goods correctly.

If you do not care about the incorrect vendor on the invoice attached to inventory transaction or the GL showing the correct vendor, you could do AP Invoice CM for the original Invoice, then do new invoice for the correct vendor.   This will only change the AP module and if the same amounts will wash the GL amounts correctly.

If you have already resolved this, please indicate how you resolved and verify so others can see how your fixed it if they run across the same situation.