AddVirtualButton requires 4 parameters.
AddVirtualButton( index, x, y, size )
and doesn't return a value (to bNoob.button, in your snippet) so you can skip the "bNoob.button =" part and put the value from it into the index parameter.
The range of "index" must be from 1 to 12, and the value is not one that is auto-generated, like with the media commands.