I believe the problem is that the command is being called before the window is created ( i.e. viz.go() ). Try changing your code to the following:
Code:
viz.window.setPosition(500,500)
viz.window.setSize(700,300)
viz.window.setBorder(viz.BORDER_NONE)
viz.go()
viz.window.setFloat()
This will be fixed in a future release though.