We are setting up ACH to pay vendors. Any advice on how I can test this or for setting Electronic Bank Payments in general? Also, most of our payments are vendor prepayments.

Talk to your bank rep. They should be able give you a format you can compare against and/or a place you can send a sample file into to confirm that it matches their specs. If available, setup in sandbox to send sample file to bank to confirm and test full payment process from beginning to end with your AP folks. Hopefully everything works fine out of the gate fingers crossedbut if not, adjust the template, EBP setup, or your process as needed to make things work.