Are you sure about the rotation lever being used to hold down the boom? I've never seen a rotation lever that would be strong enough to hold down the boom! It would have to be a monster!!

The only purpose I can see for the vang is to induce rotation in the mast when sailing broad. I don't find this necessary as the mast rotates anyway with pressure in the sail, and when the cat rigged boats go downwind (on the runs) the sail is stalled anyway so the rotation doesn't matter at all.

The mainsheet holds the boom down.

The most useful thing a vang can do is reduce the weight of your boat when you take it off