I am a part time.....make that a sometime, racer so I look for correction on this from the experienced ones. My view of the the traveler and mainsheet are that main angle (to the wind) is controlled not with the sheet but with the traveler, when you are sailing to windward. Sail tension/flatness are set with the mainsheet. Except in light winds where you want some fullness in the main, or in really hard winds where you want to flatten the main beyond what you can do with a cunningham or the downhaul I have, you want almost no vang tension or lift. Only after the sheeting angle puts the boom outside of the traveler, do things change. At that point, sheeting angle is set with the main sheet, and sail set/tension is set with the vang.
I have a standard main, so the following is conjecture. The only real loss with a loose footed main is the ability to add tension under high wind conditions when sailing off the wind. This makes your outhaul and downhaul/controls all the more important. In light winds, you still can support the weight of the boom to keep fullness in the sail, and I believe that the use of traveler, sheet, and vang remain the same. Also if you try to flatten a loose footed main with the vang, as has been stated, it really puts a lot of force on the gooseneck and the boom itself. So that's my story
Happy Thanksgiving to all.
John G.
Valhalla
Commander No 287