O'day 192 Vang is a good idea
My boat came with a pigtail crimped to the backstay on which to hang the boom and a boom vang. The pigtail is not as versatile as a real topping lift since you can't use it to shape the sail in light air. I think adding a vang is a good idea. It's a fairly high aspect mainsail and the twist can get pretty bad since there is no traveller. Also, since the spreaders are swept back, using the vang to flatten the sail and limit twist will reduce the amount of chafe on the sail. So, you could argue the vang will pay for itself in the long run by helping your main last longer.
