I've looked at some old Vincent technical sheets and they don't seem to list any pipe/silencer lengths for singles.there are lenghts and pipe diameters for twins, maybe this would be a place to start looking.
Sheet 4D:-'A straight through extension pipe can be made up or ordered from the works and should extend to a point just behind the rear axle. A variety of racing exhaust pipes are listed in the Spares Book, a megaphone will reduce power a lower revs,and is not suitable for this type of engine. in any case the valve timing is not one which answers well to a megaphone. 2"(50.80mm) diameter pipes,44"(111cm) long,will give an increase in maximum power at the loss of flexibility below 4,500 rpm. these pipes fitted to twins reduce the ground clearance on righthand bends and they are recommended for sprints and fast circuits only. For short circuits 1 5/8"(41.27mm) diameter pipes 54" to 56" (137 to 142cm) long are recommended; it should be bourne in mind that separate exhaust pipes preclude the fitting of the standard kickstarter and that an alernative crank is not supplied.'
So sayeth the Works