You could probably use the Vang on a 14 to sail directly downwind, but most 16 sailors tack or jib downwind which eliminates the boom raising problem you're referring to, and gets you there much quicker as well. A bridle fly will help you identify and sail to 90 degrees apparent wind. The apparent wind would be a combination of the true wind off your stern and the wind in your face which add together to provide a new component of wind and direction. The boom position on this point of sail is generally just outside the rear corner casting, so you don't have a problem with it flying up. The VMG or Distance Made Good with this downwind tacking method generally provides much better performance than sailing with the wind directly at your back.