Clipper Lounge at the Mandarin Oriental is my best bet for afternoon tea. They have tea for two (on one of those classy 3 tiered stands - yay!) for under HKD400 (for both ppl), coffee/tea incl and bottomless

Smokers get horrible seats in the corridor though. If you're booking, try and get them to reserve a window seat.
Tea at the Pen. is overrated (scones are dry EVERY time I go there. Myabe it's just my luck...), plus on weekends you have to line up. Totally degrades them and the whole tea-ing event IMO...
Not sure about vouchers, sorry - check their websites.