Actually it just gives you a $5 vudu credit. The card says towards any of the fast and furious movies but you can use it on anything on vudu. You didn't make a mistake. I have redeemed 9 of these. You are allowed to redeem 10 total per a single account.
The prefix matters because you can only redeem a certain amount of codes with the same prefix on one account. It is to make sure we aren't maxed out on our account, but that we can still use it. Also, you didn't make a mistake; even though it says fast five on the card you can use it towards any movie on Vudu :)