My last boat was a 25 ft Cape Dory rigged with a topping lift. The best function this set up had over a rigid vang was to raise the entire boom high up out of the cockpit,(raising the gooseneck up the boom while leveling the boom with the topping life)so we could more comfortably entertain guests while at
anchor or dockside without hitting our heads on the boom. Also helped avoid hitting our heads while scrubbing the topsides, . I don''t believe a rigid
boom vang would have given us this range of height adjustment..