Unless the virtual keyboard is sending some special information with "key presses" then I can't imagine you'll able to restrict input to that "device". Text boxes take any "keyboard" input real or virtual. The way I can imagine restricting it is if you can somehow track the key input to the virtual device itself. In the scope of an ASP.NET app, you'll be limited to javascript, so I doubt it can work.
-Peter
peterlanoie.blog