You can pass the ground object as the height value:
Code:
import viz
import vizact
viz.go()
ground = viz.add('ground.osgb',euler=[0,20,0])
avatar = viz.addAvatar('vcc_male2.cfg')
walk = vizact.walkTo([0,ground,10])
avatar.runAction(walk)
viz.MainView.setPosition([10,0,5])
viz.MainView.setEuler([-90,0,0])