For lightweight 'planing hull' boats - Easing off the vang allows the mains'l to grossly "power up" when you try to come come across the wind to 'feather up' ... as sometimes such gusts will have components of backing/veering/backing. etc. wind.... my technique (in lightweight 'planing' boats) is to keep the main a FLAT AS POSSIBLE and 'blade-out' when necessary rather than 'feathering up' ..... to keep the board/keel going in a 'straight line' as much as possible.
What could you do to keep the body of the sail flat but allow the leech to twist and dump air? Keep the vang tight and ease the sheet with the traveller all the way leeward?