Boom angle
It sounds to me like you are giving too much angle to your boom and the angle of you sail to the wind. When you are on a beam reach, your boom should be no more than 45 degrees from center. Your sail's angle off the mast, the luff, is more away from center than your boom. As it comes out from the track of the mast, it has its own angle away from the mast which is more than the angle of the boom, even more away from center than the boom. When you use vang to tighten downwards, you reduce that extra sail angle from the boom, but more on the top part of the sail than the bottom. To get less angle at the bottom part of the luff of the main, some people have cunningham rigging so they can pull down. You can also tension the luff of the main by pulling up on the main halyard. No matter what you do, the angle of the leading edge of the main is greater than the boom.If your spreaders are involved with the sail on a broad reach, you are clearly using too much boom angle out. Pull it in until the sail clears the spreaders and then vang down after that.On a beam reach, start with your boom on center or close to it, then let her out slowly until your telltales stream out and straight in at least one position, then try tightening or lossening the vang until you get the whole team of telltales singing the same song.