The PAC1223 spring is well known to me, it being a progressively wound spring of ovate wire section, and is favored by many Classic racers for fitment in their Norton Commando engines, but is not used in any of the top performing, 2 valve big single engines, where valve lifts are approaching 15 mm.
In these engines, springs often used are RD10034 or PSI1536 - seriously strong when compared to PAC1223, but found necessary to maintain control of the valve train at the engine speeds where maximum power is developed. These two springs are doubles, and are progressively wound round wire items.
For use in a Vincent engine where standard cams are still used, the PAC1223 would most probably be a spring that would be hard to better