As I understand it, the tip is 15% of the "real" total spent on the meal, not on the amount you paid for the voucher. Does that make sense? That is, if the actual food/drink total is $50, the server receives $7.50 (I think the math is right). Now, you pay ONLY whatever the voucher costs you ($31?). The server's tip IS included, though occasionally, if we get good service, we'll tip extra ($3-5). Thus, the server has incentive to encourage you to use the voucher to its fullest (order the most expensive meals, etc). In general, I think it's a win-win situation for both guest and server.
TOok