Welcome to the Community. I can help you display your VAT number to invoices in QuickBooks.
You can start by enabling the custom fields from the Account and Settings section. This way, you can add extra info to your sales forms.
Please take note that this is only available for Plus and Essentials versions. Here's how:
Once done, please proceed with opening the invoice template to verify if this field is already available. You can follow these steps:
When creating invoices, please make to select the correct template. This way, the changes you make from the custom fields show up (see screenshot below).
I've also attached an article you can use to know more about how to customise templates in QuickBooks: Customise invoices, quotes, and sales receipts in QuickBooks Online.
I'm here anytime you have other concerns. Have a great day.