You can set a default expense account for the vendor under the GL Accounts Tab of the Vendor Profile, so when you enter a line in an AP bill for that vendor, the default Expense account (and subaccount if you are using those) can populate for you.
If you are trying to record a direct payment of a bill (i.e. the bill and the payment) in the same transaction (since you mentioned “cash in bank” as one of the accounts) take a look at entering the bill using the “Quick Check” screen to record the expense and payment together and make sure you select the correct bank account at the top.