On the 20' C scow that I sail when we get into the heavier winds we crank hard on the vang and cunningham, let out the traveler and work the mainsheet to spill any excess wind. You want to flatten the sail as much as possible and keep the power low and forward. A tight vang, and cunningham will help flatten the luff of your sail, keeping the boom down. The leach is then controlled by main sheet tension. The C scow is a 20' dinghy and we can't reef. I would think the vang and cunningham would have similar effect on keel boats and rigs that can reef. By letting off the vang as mentioned by SanderO/jef you would actually hold more wind/power. The only time we let off that vang is if we are getting tipped on our ear and a strong helm developes, we release the pressure, flatten out the boat and then bring the vang back on again.
This worked well enough to secure 3rd in our 24 boat fleet for this season.