There was a thread about this recently but I just can't find it. Anyway here is the full quote from the VS website
We've made another enhancement to booking flights on this website. Now you can buy a flight from most departure points and pay for it with a UK or USA credit card. So if you live in New Jersey and want to book a flight from Hong Kong to London, you can!
All you have to do is select your billing address country on the same page you fill in your passenger details.
There are still a few departure points that remain restricted, these include flights departing from Nigeria, China (Shanghai only) and India.