I have a vanilla gift card Visa gift card and it will only let me activate it

If you are trying to redeem a Vanilla gift card to your Apple Account, that will not work; you can only redeem Apple gift cards. It is also unlikely you can add it as a payment type since it is not a true credit or standard bank debit card. You would need to go a store where you can use it to buy Apple gift cards and then redeem those to your account.