start at wall. measure off of wall and up the vault 4' 1/8" at each room corner and then pop a chalk line across the vault ceiling. then measure along that chalk line back down to the wall in different spots to determine if the wall is straight or not. if the wall is straight you are good to go, if not then adjustments will be needed, figure all this out before starting
next, measure up to the peak of the vault from the chalk line at different spots to determine if that is a consecutive measurement and if not adjust accordingly along with any pre determined wall adjustments.