How legal is it to buy virtual cards without linking them to documents?
Loaded question.
Here in the US it's 100% fine.
No different then walking into your local Target WalMart 7-11 whatever and putting cash on the counter and buying a prepaid Visa (or MC or Amex or Discover) gift card.
They make great last minute gifts :-)
In other parts of the world I have no idea what the rules are.
However, I have traveled to a few countries and bought them with cash with no ID and used online with no ID so I really don't think it's that big a deal.
There are probably limits to the amount of money you can do this with before there is some legal paperwork that has to be filed but I have no idea what it is and it would vary from country to country anyway.
-Dave