Topcat:
I believe they get their tip based on how much the food costs, not how much you paid for the voucher. That is why so many of the waiters encourage you to get the most expensive items, as it boosts their tip. You are, of course, free to leave more tip, if you feel it necessary. i.e. you had great service, etc.
I just mentioned the fact that the tip was included because one time I let my husband sign for the bill and he didn't realize the tip was included with the vouchers and he left a BIG tip based on the whole meal costs. I personally didn't think the waiter even deserved the tip he got with the voucher, let alone the extra. But, I digress.
Maggie