bad advice
Using a boom vang as a preventor is something of a dinghy trick, but if you're running anything larger than a Laser or an FJ it's a great way break equipment because suddenly the vang is doing the work of both the vang and the preventor. That's WAY too much pressure for the toe rail, the vang blocks, the boom bail, et cetera. Sure, you might get away with it in light seas and light air on something as small as an H27, but think about what you're asking the rig to do at that point: a preventor controls the boom along a horizontal arc, while the vang controls the boom along a vertical arc. Now you're asking a single piece of gear to do both. Not smart.