If your avatar is cycling an animation you can use set its speed to 0 to pause and 1 to resume the animation using setAnimationSpeed.
Code:
import viz
viz.go()
avatar = viz.add('vcc_female.cfg', pos = [0,0,10], euler = [180,0,0])
avatar.state(2)
vizact.onkeydown('1', avatar.setAnimationSpeed,0,0)
vizact.onkeydown('2', avatar.setAnimationSpeed,0,1)