The frontier between ''vintage'' and ''junk'' ?

Once person's vintage may be another's junk, and vice-versa

Not really, not in absolute terms. A vintage product i.e., one worthy of mention, is one that has something innovative or exceptional or rare, or...
TTs with outstanding and copious engineering, speaker drivers with powerful magnets, electronics with excellent design...
And then, there is stuff that had low intrinsic value even then...