The vang or 'Kicking Strap' keeps control of the boom and the sail volume. If it's not fitted, or too loose, the sail becomes inefficient by twisting.
When your setup, pull in the mainsheet all the way and then tighten the vang, this should be considered the normal position. Some people mark each strand of the vang sheet to easily find the 'normal' position. In a light wind, leave it a little loose, just enough to prevent the boom from lifting.