Mostly I traffic in electronics and travel deals. Sometimes entertainment/leisure stuff. So this is a bit strange. But if you're interested in Victoria's Secret, read on ...
On the surface, this is an in-store promotion. But like most in-store promotions, there's a "no purchase necessary" component. (I covered a similar promotion they ran last year HERE.)
The "official rules" of the contest going on now can be read HERE.
But in doing a Google search for the topic (surely someone must have written about this already!), I came across this summary of the deal on a blog called Free Stuff Finder. You can read their summary of the deal HERE, so that I don't have to do much work myself!
I think the key is that you are guaranteed to get a $10 promotional card (could be more than $10) for every entry that you mail in. Also key is that you EXACTLY follow the rules, in terms of the 4x6 paper size and #10 envelopes. Finally, realize that you can only use these promotional cards during a short time frame from early- to mid-December.
If you read the notes of the Free Stuff Finder blog, you'll see speculation (that I think is probably correct) that you can probably only use one of the promo cards per order. This is almost certainly true with online orders. But it may be a YMMV situation in-store, where you may be able to convince an employee to let you combine at least a few cards.