I would avoid using the vizinfo feature for detailed control over GUI elements.
Try manualy aranging the gui. Here is some sample code. Because the elements are under the viz.ORTHO parent, there positions are specified in pixels.
Code:
guiRoot = viz.addGroup(parent=viz.ORTHO)
viz.link(viz.CenterTop, guiRoot)
buttonRoot = viz.addGroup(parent=guiRoot, pos=(0, -100, 0))
button = viz.addButton(parent=buttonRoot, pos=(200,0,0), scale=[1]*3)
label = viz.addText('Toggle HUD', parent=button, pos=(-50,25,0), scale=[25]*3)
button = viz.addButton(parent=buttonRoot, pos=(0,0,0), scale=[1]*3)
label = viz.addText('Set Background', parent=button, pos=(-50,25,0), scale=[25]*3)