The piazza was created in 3ds Max with lighting baked into the model. One way to create a scene that has both day and night is to have two versions of the same model, each with it's own baked lighting.
To darken the piazza in Vizard you could try blending in a black texture as described
here or use a post process effect:
Code:
import viz
viz.go()
piazza = viz.addChild('piazza.osgb')
import vizfx.postprocess
from vizfx.postprocess.color import ExposureEffect
effect = ExposureEffect(-2)
vizfx.postprocess.addEffect(effect)
There's a night version of the sky you could swap in as well:
Code:
piazza.visible(viz.OFF,node='pz_skydome-GEODE')
viz.addChild('sky_night.osgb')