That geometry seems no worse than for a DS.
If your boat is using end boom sheeting like the one in the picture I could see the value of adding a vang. If your boat is already set up with mid boom sheeting the vang is probably only adding minimal performance.
OK, let's think that thought to its logical conclusion. The ideal sheeting position should then be next to the mast.
Clearly, if the sail pulls in a given direction, then the boom will move out and up until the sheet pulls in the opposite direction to the sail force. The direction of the sail force would, of course, depend itself on boom position and angle, but disregarding this for a moment, the center of the boom is closer to the centerline of the boat, which gives mid-boom sheeting a more vertical pull.
Upwind, the closer the boom gets to the horizontal and amidships position, the less difference there will be in the angle of pull of the two sheeting arrangements in absolute terms.
Downwind, there should be a bigger difference, but it could be that the situation is already so far from optimal by then, that sheeting position doesn't matter enough. That could account for the fact that many people, including racers, continue to sail with the bridle or rope traveler in the rear, but if they want to see improved performance they get religious about their vangs, like K.C..
I'm (still) vangless - but then I have a non-standard configuration at the mast partners that makes fitting a vang particularly challenging, and I need to puzzle out how I deal with that before I can proceed further. Reading the discussions has me convinced it's worth to put that on my list of possible future upgrades.