Re: how to use vang?
The purpose of the vang is to reduce twist in the main. As the sheet is eased, the aft end of the boom lifts and the top of the main "twists". Some twist in the main is good since wind speed is higher up there and the true wind will come from more aft, but excessive twist is bad. Going upwind you can reduce twist by easing the traveller instead of the sheet, but as you head further downwind you get to a point where you can no longer let off the traveler and have to let out the mainsheet. As you do this, cranking in the vang will flatten the main. You can also use the vang upwind to further flatten the main, but I primarily rely on playing off the traveller against the mainsheet for this purpose when heading upwind, and use the vang downwind.
In much wind at all, you need to set the vang before you ease off the mainsheet. There's just too much force against you if you try to flatten the main once it's eased well off.
1977 C&C 30 Mk 1 hailing from Port Clinton, Ohio