If a vendor falls under one 1099 income category only, is the 1099 report as simple as showing the bank payment transactions associated with the vendor during the year?

I believe most 1099 reports use the vendor bill and the expense account on that vendor bill. Not the actual payment

That's what confuses me. It sounds complicated. Why don't we just use a custom 1099 category vendor field for that? I think defining the 1099 category per GL account is only applicable if a single vendor has multiple 1099 categories or not all payments to the vendor are considered as part of the 1099 report. But even so, payments are not associated with specific bill lines, so there must be some allocation of payments to make the report accurate.