Okay. I have a US iTunes account. I set it up using the Vanilla Mastercard method. Plus a friend sent me a $100 iTunes gift card.
I stupidly redeemed the gift card before using the balance on the Vanilla Mastercard.
I currently have a $50 balance on the Mastercard which is now dwindling $2.50 per month in "service charges". And I have $50 left on the iTunes gift card balance. I have about $50 of purchases that I want to make. However, by default, iTunes takes from your gift card balance first and your credit card balance second.
If you purchase iTunes gift cards, it charges ONLY your credit card. So I purchased a $50 gift card on iTunes, emptying the Vanilla Mastercard, and bumping my iTunes store balance to $100.