Boom vangs mostly exist (in this type of boat) to hold the boom down and control main sail twist. When running deep reaches and runs, you do not want the boom to lift. Also, allowing the boom to lift allows the top of the main to twist off too far and create a sideways force (this is the cause of "death rolls" in small boats like the Laser.) In smaller boats with bendy masts, a very powerful vang holds the main flat, and acts like a ram forcing more bend in the mast, which also depowers the main. The most effective angle for the vang is about 45º to the boom, and it should be fixed at the same pivot point as the gooseneck. My vang attaches near the base of the mast to a D bail that is through bolted to the mast. The reason for this is that the downward pull of the vang doesn't change with the angle the boom swings out. A notable exception to this is the Star class sailboat, which has a circular curved track on the deck that the vang attaches to. This provides A LOT of down force on the boom, while allowing that force to be equal through the swing radius of the boom. On my 192 with Dwyer mast, I have a sliding gooseneck that is not fixed position. As such, I have a line attached to the bottom of the gooseneck and then to the same bail as the vang. This downhaul line is similar to a Cunningham, in that it pulls down on the luff, controlling the position of max draft in the main. Pulling on downhaul (or Cunningham) moves the point of max draft forward, which is important in reducing weatherhelm in strong winds.
Blue line along mast is downhaul, grey line is vang. (Yellow is spinnaker halyard, red is jib halyard, and green is main halyard. Some have complained about my multicolored approach to line choice.

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Another thing I do with my vang is to have an easily adjustable topping lift. I can balance the boom between the topping lift and the vang, so called vang sheeting. Because the 192 (and your 19) are end boom sheeting without a traveler, pulling too much mainsheet to pull the boom in for sailing to windward pulls too much leech tension, and pulls all the twist out of the main. In lighter wind, it's important to have the top of the main twist out more than the bottom, because of different wind velocities and apparent wind angle going up from the water. In many cases of light air, the weight of the boom can even pull out the twist, so I use the topping lift to lift the boom and induce twist, and lock that position with the vang. This allows the angle of attack of the boom to be adjusted by the main sheet without affecting twist.