I think officially the definition of vintage is either 40 or 50 years and antique is 100 yrs or older. That pretty much works for me. My definition of vintage stops when plastic became the dominant material used, I would still call some machines made into the early 80s vintage. Also consider the style and construction of machines. Machines from the mid 70s and earlier are primarily metal in construction. After that point more and more plastic found it's way into machines. The problem with machines that have plastic gears from the 60s and 70s is the usable lifespan of the plastic is at it's end. The plastic gears are starting to fail and aren't always available for replacement. That's one of the reasons so many of us here favor machines with all metal construction.
The cheap modern machines you can buy now are completely different beasts with plastic clamshell bodies and ether stamped metal or plastic internals everywhere the manufacturers can get away with it. They're built just well enough for your warrantee to wear out (if you're lucky and only give them moderate use). The better quality new machines aren't as bad. Another big difference is sewing was accomplished mechanically on vintage machines. These days it's more likely to be computer controlled servo motors doing the work.
Rodney-king of long winded answers