Prior to sending a payment to your vendor you can easily add their payment method, or request them to do so on their own.
You have two options for adding a new payment method.
Ask your vendor for their most up-to-data account and routing numbers. The "Update payment method" button on the vendor page should allow you to update it (see below).
If you have the payment method information ready, you can add it by clicking the "Vendor info" tab.
Here you will find options for:
"Add bank account" - Adding a new bank account for ACH (Direct deposit) payments
"Add address" - Adding a new address for sending a check