Go to the Edit menu, to Links, and then to Fields and press Enter.

The Files on the report will be displayed. If you press Enter again, you will be able to see the Linking field between the 2 files.

The linking field between the API????H and the APVEND.R should be Vendor number.

You will have to change the 'link map', as you are correct in not being able to link batch detail directly to the vendor file. The links for this report should be defined as follows, which you can access from the Edit menu, and Links:

Driver: API???D

The driving file for 99% of all reports will be the most 'detailed' file. This makes reports run faster as well. Unless the purpose of your report is to list all vendors, regardless if they have an invoice in that batch - not likely - then you will have to change the report driver.

Files: API???D to API???H
API???H to APVEND.R

The batch detail links to the batch header, and in turn the batch header links to the vendor master.

Fields: API???D API???H Batch/trans number
API???H APVEND.R Vendor number

These are more appropriate choices than 'GL account'.

Link mode: One-to-one

When your most detailed file is driving the report, this should be the link mode, which will speed up the report a bit.

When these changes are made, I trust your report will be working much better.