Could be added in address fieldset or to a new "Fiscal Data" fieldset.

VAT and Tax Code are simple strings to set in the customer and company forms to be printed in the invoice (like address and phone).
In some countries, they are necessary information to be shown in invoice in order be a valid fiscal document:

Some information:
VAT Code is a common concept in all EU: http://en.wikipedia.org/wiki/VAT_identification_number
Tax Code it seems to be similar but different in each country: http://en.wikipedia.org/wiki/Codice_Fiscale or http://en.wikipedia.org/wiki/Taxpayer_Identification_Number or http://en.wikipedia.org/wiki/Tax_code_%28PAYE%29 or etc...

Eg.: In Italy, Tax Code (declined as Codice Fiscale = Fiscal Code) is mandatory if customer is a private entity (hasn't a VAT Code).