To create order rules, you need the Order Rules module.
The PopulateVATNumber order rule action populates a VAT number on an order when it doesn't have one or when you want to override the existing value. In the Param field, enter the VAT number you want to add.
The PopulateEORINumber order rule action does the same as above but instead of populating the VAT field, it will populate the EORI field on the order. For the Param you would enter the EORI number you want to add.