Hello, Rigo. I'm here to help.
You're correct. While QuickBooks Online (QBO) doesn't automatically create a separate Vendor Credit for the bill overpayment, it still stores the Unapplied Payment as a credit which you can apply to your next open bill, as in the screenshot below.

If you've communicated with your vendor to store your overpayment as a credit on your open bill, you can mark the open bill as paid using the Unapplied Payment. What this does is that it links the bill payment transaction to two of the bills. I'd be glad to assist you in accomplishing this in QuickBooks Online (QBO).
To apply the credit to your next open bill, you can follow these steps:
- Choose Expenses on the left navigation panel.
- Select Vendors.
- Click on the vendor profile where the open bill exists.
- Locate the open bill and choose the drop-down arrow.
- Choose Mark as paid.

- Scroll down and ensure placing a checkmark for the Unapplied Payment # [no.].
- Hit Save and close.
If you want to run expense or Accounts Payable reports after this, let me add this article as a future guide: Run reports in QuickBooks Online.
We'll be here in the Community if you have further questions about handling vendor overpayments in QBO. We'll do our best to assist. Take care!