Actually, they don't take them away.

Vouchers are sometimes (almost always) given as a reward for purchases at 3CD.  Typically, the size of the voucher depends upon the size of the purchase. I have no idea what the "formula" is for determining the size of the voucher, but since I also am a purchaser as well as a vendor, I can see that it is somehow proportional to the total purchase cost size.

They do expire if you don't use them soon enough. They are good for 30 days, but I'm not entirely sure of that time period, since I never use mine.  (I don't use mine because it takes money out of the pocket of fellow vendors. Purchases of items with vouchers are made pretty much at the expense of the vendor.)   ... But, be sure to use yours when you get them -it's part of what we vendors give back to the Vue community.

 Vouchers also are given as prizes for the monthly Theme contests - for winning compositions and pictures. They may be given for other promotional purposes as well. 

  Also, C3D gives spot sales to large numbers of customers randomly selected, where products chosen by the site administrator are often marked down 85 - 90% from their normal purchase price. (Again, it is the vendors, rather than the site who are mostly bearing the cost of  this program.)

 By the way, there are well more than 1,500 items (I think it's now in the neighborhood of 2,000 items) that are eligible for purchase with a voucher.   The reason that there are not more is that some vendors simply can't afford to participate in the voucher program. Many of us are doing this for a living, and we all sometimes struggle to feed the children, ... you know how it is!