The classification of a model should be made not on the age of the vac, but when it was first launched. Using ChesterMike's definition above, the Senior 652 which was first seen in 1959 would be classified as vintage, but the same model ran up until 1974 when the Ranger replaced it, so later 652 models from the 60's and 70's would be classified as Classic, even though they look the same as the earliest models from 1959 - in the vintage period.
As far as I'm concerned - all plastivacs should be classified as contemporary, be that a Dyson Dc01 from 1993 or an Electrolux 500 from 1973. Most metal bodied vacs with cloth bags should be classified as Classic period vacs, and the really old Pre-War 1930's vacs and older should be the only ones to be called true Vintage.
Look at how we define modern, classic and vintage cars - would we call an MGB roadster a vintage car? would we call a Morris Marina a vintage car? would we call a Porsche 911 a vintage car? No, we would call them classic cars, even though they came from the 60's and 70's. What is pictured below is what we call a vintage car.
Vintage is from the pre-war period (before 1945), and this should apply to vacuum cleaners as well as cars.
A "real" vintage car:
