Foot switches allow you to press the left/right mouse buttons using your feet (thus keeping your hands free to play the instrument).
If you want to be able to go forward and back, you will want a 2-button foot switch. If you only need to go forward (PageDown), a single button (the left mouse button) switch will do. Either way, you have a few options.
Home baked solution
If you're not afraid of using a soldering gun, you can make your own switch by sacrificing a USB mouse. That's what I did. I already had two sustain pedals (the normally open type) and a BlueTooth mouse (can be found on Ebay for a few dollars), so I bought two 1/4" jacks and soldered them to the mouse button switches on the mouse PCB. The sustain pedals can now be used to move back and forth through the song. Thanks to BlueTooth, there are no wires between the pedals and the laptop.
None of these have been verified to work with VirtMus but from the description it looks like they might. If they act as a mouse and provide mouse-click events they should work.
http://www.bilila.com/ sells a foot pedal that can be used to "turn pages". Be aware that this device sends PgUp/PgDn keys to the PC so VirtMus will always scroll a full page when the pedal is depressed. In order to scroll by less than a full page, the device must send left/right button mouse clicks. See:
http://www.delcom-eng.com/ sells a number of USB foot switches that generate mouse click events. See for example:
http://www.kinesis-ergo.com/ also sells a few types of foot switches that generate mouse click events. See for example:
http://usb.brando.com/ sells 2 and 3 button foot switches: