Re: main sail blown?
The vang is not there to bend back the mast - that's way too much work to ask of it, and more importantly, the fittings that you would be asking to work against that leverage. If you want to bend the mast by pulling the sail down, then crank in on the sheet until it's in all the way, and then snug up the bang, but to try and accomplish that with a rope vang alone in heavy winds is a lot of work for you and your equipment.
The vang is there to reduce twist in the sail. We want to do this, because the lift that is generated by the sail is pretty much perpendicular to the surface of the sail, so when sailing off the wind and the boom lifts, the lower portion of the sail is looking at the water, and that's where the lift is pulling the sail, inducing heeling of the boat. By reducing the twist, that lift is now directed more horizontally, where we want the boat to be pulled. But it's not going to do much for you on a close haul, because your sheet is a more effective lever arm for flattening the sail. At some point, the sheet starts pulling mainly down and not in, and that's where it can be helpful, especially in lighter winds, to switch to the traveller for sail trim, as it's pull is strictly horizontal.
Flo, from your pics, I'd say the sail is pretty blown. One clue is that your leech line is cranked way in, which suggests that the cloth is pretty stretched (if you can loosen it and keep the leech from flapping, go ahead. That curl is making an uphill climb for the wind exiting the sail, and causing lots of turbulent drag. Get some yarn and stitch a couple telltales in there and you'll see what I mean. If the cloth isn't too worn, the sail can be restitched. I'm not sure what your boat is, but I have a pretty decent Catalina 27 main that came with my boat. The problem is that the PO spilled some West Marine resin on the area near the tack, so it's discolored and still a bit sticky, but I can give it to you cheap, and an hour or so with some vinegar or alcohol should cure the stickiness. I'm over in Sausalito, PM me if I can be of any assistance -