When a vendor record does not have a default payable account set, the account used in the lastest vendor bill created in the system will be used on the vendor bill. I'm not 100% sure, but I think the only way around this would be by setting up the default payables account on all of your vendor records... unless somebody else has a better idea. At the company I work for, we have a default payable account for all of our vendors and we don't run into this issue.