Hello, RafalLukawiecki.

Thank you for sharing your concern here in the Community.

We can add the VAT number using the Custom field. This is a field that you can use to add anything to your invoice template.

Here's how:

1. Sign in to your QuickBooks Online account.
2. Click on the Gear icon.
3. Select Custom and styles.
4. Click on the drop-down arrow in the New style tab, then choose Invoice.
5. Select the Content tab and click the invoice template on the right side (header)
6. Scroll down and look for the +Custom field.
7. Put a checkmark on the tick box, add the VAT number and click done.

You can also read this article in Import custom form styles for invoices  

Feel free to reply to this post if you have additional questions or concerns. I'm always here to help.