An account per vendor isn't a feature. As a result users break up payment runs with a few methods.
1. By payment method. Add it as a filter for the vendor section of the suggest vendor payments.
2. Priority. Another filter you can have.
3. As Dirk said. A journal batch per bank account. You would then need something on the vendor to split them up as a filter. You could use "Vendor Posting Group" if you don't have any other grouping logic right now.
