Did one Fastace shock give sufficient damping on the rear ? I mean they are marketed for much smaller bikes than the Vincent...On the rear it really should have one either side, this would work well with shrouds to hide the springs..
Would 2 dampers be to much ? The rebound can be adjusted but I guess the compression cannot (?).
The Fastace shock is also available in 10" = 255 mm, which is close to the lenght of my rear AVO coilover and might be better on a roadbike.