Hi the two vangs ( wangs ) on a full size barge control the 4th corner of the sail, and the sheet end of the tops'l by acting on the outer end of the sprit. The 2 wires have a tackle at the bottom, which comes to the deck very near the main horse.
In a model barge, if you put a functional block at the outer end of the spreet you can replace the 2 vangs with one continuous string, and you only need one servo. Make fast on deck just forward of main sheet horse, run up to end of sprit, through block and down to corresponding position on the other side, i,e just forward of horse, where is goes down below decks and to the servo with a simple arm to sheet in/out as required, in line with main sheet position on another servo.
I would reccomend 'Sailing a Thames Barge Sail by Sail' very cheap, from TBSC sales officer.