Roller reefing was an awful system, almost universally used in the '60's and until what we called jiffy reefing came along. It was possible to use a boom vang on a jiffy reefed sail, however. You used a device that was called a reefing claw, basically looking like a circle with an opening. A line
was run from the point on the claw opposite the opening, and there were rollers of some kind on either side of the opening to minimize chafe on the sail. Look up reefing claw for an illustration.