Actually, I've found that having a Vice Marshal actually hurts more than helps. If your Marshal gets stabbed/captured, that army is stuck on his last Marshal setting until he comes back, as the Vice Marshal cannot change the settings. Sure, you still have a Vice Marshal enforcing line settings, only now they might be the wrong ones. I've actually seen incidents where the Vice Marshal were told to move away from the battle site so that he can't screw up his army's settings.
It's another example where the game mechanics ended up discouraging what was meant to be encouraged (more positions, more distribution of power, etc.)
