The simplest solution solution would be to buy a small mouse. That'd give you 3 buttons which Vizard can easily read. Glue two down if you only want one. Just about all mice these days are USB, so simply go buy a couple USB extension cables (16 ft. limit some folks say about USB).
Alternatively you can build your own button boxes and interface them with a LabJack (or similar) A2D/D2A acquisition device:
www.labjack.com
Please contact
support@worldviz.com about a Vizard driver for the LabJack, in case you decide to go for this solution.