Do I need to have a VAT number to use the Envato Market affiliate program?
You are Author in Envato Market. Asuming that you have submitted W8 form. If you submitted with your TIN number then no need to submit it again in Imapct site. Your Envato submitted information will use in Impact.
For more information or getting official answer you can contact with Envato Market Affiliates Team @emile_b (email@example.com) or firstname.lastname@example.org