I too have already purchased the vouchers (starting to think that was a mistake now).
I've looked at the terms and conditions of the Shareholders Card, and it says you can't use the discount to buy meal vouchers, and can't be combined with other offers. However, the vouchers can't really be classified as an 'offer' as such, because they don't really seem to be saving you much money (if any) in some restaurants.
I have sent the shareholders club an email asking for some clarification on this, and will hold off buying my shares until I get a response.