Pete, Don't think that's the reason the gearknob overdrive switch wasn't applied to the Vitesse. It just wasn't around at the time the Vitesse was introduced and only appeared, on other Triumphs, at the end of the Vitesse production. It was applied to the later 1850 Dolomite, the Vitesse replacement, which had the same 3 and 4 ratio as the Vitesse.
Dave