Geoff Moore, Shore Sails Ltd
...... The vang, traveler and mainsheet all work together to control twist. Adjusting twist is the fastest way the skipper can affect the balance, or feel of the boat. Less twist should result in more rudder pressure. In light air this is very difficult to detect. In heavy air the traveler is not long enough or easily adjusted enough, so we switch to vang sheeting. Vang sheeting sets a certain twist by holding the boom down hard while the main is adjusted to keep the boat balanced ......

49er.org
Summed up, the control technique with these rigs is:
...... The optimum leech tension for twist offwind, or vang sheeting to windward, is set by the vang. Use of these three controls enables any sailing technique to be accommodated. Crews who prefer vang sheeting will use less downhaul and more vang, and sheet direct from the boom - this technique gives fastest response with least sheet tension ......


North Sails Jet 14 Tuning Guide
...... Many skippers in the Jet have eliminated the traveler, relying on vang tension to
keep the main trimmed properly. With vang sheeting you tighten the vang to keep the boom down and the tension on the leech when the mainsheet is eased. For sure vang sheeting eliminates another the need for the traveler....
...just be sure you have a powerful, easily adjusted vang....
.

THISTLE TUNING TIPS von GUS Sails
...... As the wind increases and you are forced to depower the main to control heel,
use a combination of traveler and vang sheeting. Ease the traveler to a setting for the lulls then snug the vang and plat the mainsheet in the puffs. Continue to "change gears" with the traveler so that the load is distributed between the vang and the traveler ......


Shore Sails - COMET TUNING GUIDE
...... When rigging your Comet, we feel that you must make an effort to KEEP IT SIMPLE.
The following is a list of the main speed controls, in their order of importance:
1. Main sheet tension/Jib sheet tension.
2. Vang (if vang sheeting), or Traveler w/normal sheeting.
3. Mast ram or Lever.
4. Cunninghan.

"Play your vang around the course"
By Dave Dellenbaugh, a two-time America's Cup veteran
"Play your vang around the course" By Dave Dellenbaugh, a two-time America’s Cup veteran In the old days, the boom vang was used primarily on windy runs and reaches to keep your boom under control. Now it is used for many different purposes, both upwind and downwind. Here are a bunch of tips for improving your vang technique ......

Many boats use a technique called “vang sheeting” when sailing upwind in a breeze.
If you don’t vang sheet, one problem you have is that when you ease your mainsheet in a puff, you allow the leech to twist and you make the mainsail fuller (because less leech tension means less mast bend). This is not good in a puff ......
You could prevent this by dropping the traveler to leeward instead of easing the mainsheet. However, on many boats the traveler is not easy to play or it doesn’t have enough range. This leads us to the vang. By using a tight vang (i.e. vang sheeting), you control mainsail twist and shape with the vang. The sheet then controls the lateral position of the boom, which is quicker and easier than using the traveler ......

"Moore on Mainsails" - by Geoff Moore - Shore Sails LTD RI
...... Many boats have booms strong enough to utilize the boom vang instead of the traveler in heavy air. This technique is called vang sheeting. Vang sheeting works best when the traveler system is either too short or too difficult to control. In very puffy conditions there just is not enough time to play the traveler and the sheet. So the vang roughly sets the twist and the mainsheet is used to keep the boat balanced ......
Traveler sheeting allows you to be much more precise, but it slows down the reaction time. Vang sheeting is very quick, but it is a crude way to adjust your twist. Make your decision based on which priority the conditions warrant. Often, a combination of both methods works well ......

"Sophisticated Design"
...... Optimum control with minimum sheet load is obtained with “vang-sheeting” for both mainsail and boomed jib ......

Quantum One, Design Tuning Guide
...... used properly, vang sheeting in gusty winds or on smooth water will increase acceleration and maintain a constant angle of heel ......

Mike Visser, Olympia Trainer des Island Sailing Club
...... As the wind increases and you’re looking for ways to de-power without loosing performance, try “Vang Sheeting” ......
...... This is nice for two reasons: First, if the wind is puffy and you need to use the mainsheet to de-power but aren’t to the point of reefing, vang sheeting can eliminate having to pull the mainsheet down when pulling it back in from a puff. This saves effort. Second, by keeping the leach tensioned with the vang, the forward driving force of the leech is retained while you ease the sail, while the heeling power is reduced.. ......

Sailnet.com
"Mainsail Controls for Performance", Part Two by Dan Dickison
...... Two controls are integral to the rig that supports the mainsail the backstay and the vang ......
...... On board many keelboats in these conditions the vang is regarded as the throttle, and the person trimming it needs to be in constant communication with the helmsman to keep the boat on its feet ......
......Like backstays, boom vangs range from line-purchase systems, to mechanical devices, to hydraulic cylinders. There are various factors that will help you determine what system is right for your boat, but certainly having a system that can withstand the loads is a principal consideration


"The vang in a breeze" by Paul Shipley vom Washington Yacht Club
...... The reason that it works is that the top third of the sail twists off and provides little heeling force (or drive for that matter). Now that the bottom 2/3 of the sail is doing all of the driving, you get a much better lift to heeling force ratio (again, think of a lever). This effect is especially noticeable when it is starting to blow like stink .....


Segelmacher Fast Frosties from Fleet Nine by Peter Follansbee
...... Twist is controlled through the vang. This system requires both a powerful vang and a stiff boom, but it is particularly effective when it breezes up, since it allows the main to be quickly depowered while maintaining leech tension ......

"Heavy Air Mode" vom Pontchartrain Yacht Club
...... 1. Rake/tension must be correct
2. Jib sheet tension is critical for good speed
3. moderate to heavy vang tension "vang sheeting" is the key ......


By Derek Snow (1980 World Champion)
...... Since forestay sag was controlled by rig tension 'vang sheeting' could be used so that leech tension and boom position could be controlled separately ......