I don't think there are any strict rule for the term vintage. There is a general way about things being new, then old, then sort of old enough to catch interest again. Nothing is as old and out of fashion as 10 years ago. I have noticed young people in their teens and early twenties take a second look at the 1980s and borrow a few things, in the 90s it was something from the 70s.
When it comes to sewing machines, I think there is a point in 1960s when things start to get very plasticy and even flimsy. Not all machines though, there are very good machines from the 70s and eighties. Plastic was used widely by the 1950s (Elna and Bernina among them). Odly enough, the machines accused of being plastic, light weight and weak in the 1960s, come of as sturdy and capable by todays standard. Sewing machines have probably never been as cheap as today, both cost and quality (the lower price ranges that is).
The term vintage for me is something borrowed from wine making, something that has had time to mature and come into it's own right. Either way, when it relates to sewing machines it has a lot to do with a durable and dependable quality, worth repairing and fixing up, capable and well made. Not all the old machines where the best either. The eariest electronic models was introduced in the late 60s, the first computerised models in the 80s. The all mechanical machines with cams controlling the stitch patterns aren't made anymore. I don't think there are anything like the Elnagraph on any of the newer model. Does anyone have a Bernina 1008 to check with? It's listed as a mechanical machine, but I think it doesn't have the cam stack like the old.