Vines do not cushion the fall of the mobs. The mobs also can not climb the vines. In MCPC for example mobs can climb the vines to get out of holes.

Steps to reproduce:

1 - Build a vertical tube
2 - Place some vines at the bottom of the tube.
3 - Spawn some mobs at the top of the tube.
4 - See that the mobs pass straight through the vines and take fall damage.