All payments that sync with Viewpost are drafted against the Default bank account registered in your Viewpost.com account.
The banking account selected in QuickBooks Online when creating the payment is NOT the bank account the payment is sent from.
At this time, only one bank account can be used when syncing payments. If you have more than one bank account registered on Viewpost, all payments are sent from the Default Payment account. The Default Payment Account can be changed on Viewpost by selecting "Modify Defaults" on the Payment Methods page.
You must have a registered banking account on Viewpost to sync payments. To learn how to register your bank account on Viewpost see How Do I Register a Bank Account?