The Vincent was one of the very first motorcycles to come with a proper dual seat as standard. In 1948 many bikes used inferior rear suspension such as plungers, sliding pillar or the awful sprung hub. I am sure most passengers in the day appreciated what they had when on a Vincent.
Except the passenger on a standard Vincent gets virtually no benefit of rear suspension, they might as well be on a hard tail Harley.
My 250 lb barbell placed in the centre of the pillion section of the seat compressed the spring boxes 1/16 inch.
The Harley might bend as much!
Strap the bike or have someone hold the bars and try pushing down on the rear of the seat to compress the rear suspension, as you might with a swing arm bike. It's a brick.
All concepts of a lovely suspended ride quickly disappear.
T'aint there.
Glen