When creating a new vendor and linking a tax number you need to navigate to the Vat number form > create the new number > navigate back to the vendor record > in the VAT number drop-down change the view from Party to All and from the list select the VAT number for the Vendor. This process is cumbersome and could invite errors. My suggestion is that the VAT number field on the vendor record allow you to create a new VAT number, if the number does not exist ask the user 'TAX number does not exist, create now?' if the user clicks Yes, the VAT number is populated on the vendor record and a related record is created in the VAT number form, where the Country, VAT number and Company name are all populated from the Vendor record.