Hi Colin and jeff,
Hopefully the boat won't need a vang. If it will develope enough relative wind velocity, the vector will be far enough forward that a couple of feet of traveller track either side of center will be ample. Idealy this track will be located under the clew of the mainsail. If the boat turns out to be a fast sailing system, the mainsail will always be going to windward and trimmed on or near boat centerline.
Good Luck,
Bill