I had the same, I prefer to pay everything manually anyway when I get paid, it keeps me in control. But the reality is Vanquis is a credit card for bad credit. Which means they have to have some assurances that they’re going to get paid whatever they lend to you. A direct debit is the best they can put in place I guess!
Have you considered another credit-building card? There is a tool online that will give you an idea of how likely you are to be accepted for different ones, without affecting your credit rating: https://www.moneysupermarket.com/credit-cards/credit-builder/
This is strange as when I got mine in December, when talking to them I asked about setting up a direct debit on the phone and was told to wait for the card and setup up on the online account
Aqua also do credit building credit cards in the same way as Vanquis if you want to try another provider. Same insane rates, but as long as you pay in full each month that doesn’t matter.