How do I buy a runescape membership using a vanilla visa card?

I reacently got a vanilla visa card with 64 dollars on it I tried to use it to buy a runescape membership but it keeps saying payment declined. I registered the card on the vanilla visa website but all it asked for was my zipcode so I don't know if I had to do anything else. Afterwards, I went to the website to try and do it again but it said the same thing. I would find it very benificial for someone to give me some step by step or just plain old help b/c Im ready to play rs again!