Of course you need a boom vang
Pat: There are 4 things you are adjusting with all the sail trim controls for your mainsail (and jib) and they are draft position, draft depth, twist and angle of attack. The 2 controls used to adjust twist are the boom vang and the mainsheet. The most important sail trim control for twist is the boom vang. When you try to use the mainsheet as a twist control you are also effecting draft position and angle of attack. The fact that your main halyard (I think you mean mainsheet) and traveler are center mounted over the companionway has nothing to do with anything. Additionally, the only thing the traveler does is control angle of attack. It has nothing to do with sail shape.My suggestion to most beginners is to gain a complete understanding of draft depth, draft position, twist and angle of attack and then learn how each sail trim control for the main and jib effects those 4 items. Unless you have that understanding sail trim will never make any senseto you. Once you do gain that understanding you'll know more about sail trim than 75% of the sailors worldwide.As a start you should pop over to the Sail Trim Forum on this website. There are a bunch of sailors over there that will answer any sail trim question you can think of -- and some you have never thought of!!