Hold it back.
If she gets you something, you can come back with "Well, I got you this, too" pulling out your voucher, and you guys will be like this:
And if she hasn't got you anything then you reveal it at lunchtime-ish, taking a victory lap for the rest of the day.
Oh, and if she hits you with "But we agreed.." then the line to take is "I know...but I couldn't bear the thought of not getting you something for Christmas". This will lead to