The Vincent clutch requires a great deal of patience when not functioning properly. I once took mine apart 13 times in a row and eventually got it to work well, which it then did for several years without a complaint. One of the things that can cause what you describe is the 'rivet' pins in the shoe carrier fouling the back of the inner disk steel plate. Look at the back of it for telltale grooves worn in, coming off the centre in a helix type pattern, maybe and inch long. Sometime just as little as correct orientation of the plate is enough, other times a set of thin washers placed over the 9 pins is required, to give more clearance.
Having said that, once, in a fit of impatience, with a clutch getting very jumpy I ordered a Videan setup. On removing the stock setup to replace it I discovered the shoe carrier pins had come loose, The nuts on the back end have worked loose, and had worn the pivot holes on the shoes quite oval. The pins are 1/2 dia and taking the shoe holes out to 5/8" did not clean up all the wear spots, but I felt 'enough. I then rebushed the holes with ally bushings pressed in and made back to 1/2". That clutch then went in another bike where for the past several years it has done several thousand miles without a complaint.
The Videan set up is not without it's issues but once in and sorted is a fine clutch.
If you have the patience, work on the stock one, if not try a Videan one. I think available in the UK from Vince Farrell
You've got all winter to sort it!!!
Robert