Wondered that myself but scared to hookup and give a whirl.
You could give it a try..I wonder too. I have one, but it states "for guitar" I definitely does not work without a guitar. It relies on the signal to keep you back up singers on key. I would think a C chord from a keyboard would not be that much different that a c chord from a keyboard but then those keyboards are notorious for high power signal output and could fry the Vocalist 3. Maybe some day feeling frisky I'll connect my old Korg X3 to it and see what happens.