Vincents were a v twin or a sloping single, that looks to be an upright engine. That would more than likely make it a 500/650cc at the biggest. But looking closely at the engine I would have said that what you have there is a Norton model 50 (350cc) which would be mid 50s the suspension set up is too late to be 40s
Owner of Motorcycle Republic, Specialist in unfucking things that others have fucked up.