Hello there, @Marc Ogier. I'm here to help you record credit notes in QuickBooks Online (QBO) seamlessly.
To begin, let me provide some details about the process.
When you create a credit note in QBO, the transaction will only affect your customer's account not the bank register.
To help allocate the bank payment back to your customer, you'll need to create an expense or cheque transaction first. Then, select the affecting Accounts Receivables account under the Category column and choose the bank in the Payment account field. Refer to the screenshot below for reference:

Once done, link the cheque or expense to the credit note using the Receive Payment feature. When the customer's name is selected, the two transactions will automatically show up. Simply check the box beside them and click Save and close. See the snip below:

Regarding your VAT concern, you may set the tax when creating the credit note. The system will automatically adjust the amount. For more info, check out this article: Set up and use VAT in QuickBooks Online.
Before performing the steps, I highly recommend reaching out to your accountant to ensure the transactions are allocated to the correct account and avoid messing up your books.
I've also attached an awesome resource that you can access at any time. This includes a compilation of articles to provide additional guidance in performing QBO tasks: QBO Help Articles.
Please let me know how this goes. You can always count on me if you have additional questions about managing customer transactions in QBO. Have a good day ahead and take care.