They're not much safer than a physical keyboard.

The only thing I can think of that a virtual keyboard would protect you from would be a physical key logger that has been placed between your keyboard and the computer. However, if you have one of those that you yourself didn't put there, there are probably other issues you should worry about (like...who broke in and put it on there and when are they coming back for it??)

Software keyloggers usually track mouseclicks and take screenshots with every click, giving malware authors a picture of the button you're pressing with your mouse.

--Zig