Hello, usereben. 

Let's enter first the VAT number to appears in your invoices. I'll show you how to do it:  

1. From the left navigation bar, choose Taxes.
2. Click the Edit VAT drop down then click Edit Settings.
3. Make necessary changes then enter the VAT number.
4. Press Save.

You can also visit this link for more information about getting started with VAT: Set up and use automated VAT in QuickBooks Online.  

Then, you can follow my colleague's steps above to appear the VAT details on invoices.  

Keep me posted if you have any other questions or clarifications. I'm ready to help. Have a nice day!