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

Besides the condition, IMHO, it depends on whether component in question was considered to be of high quality when it first appeared.

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...

People have misused the term 'vintage' for years.Look up the definition to see what it means.

Tpreaves, I don't think anyones has confused "vintage" with Asian boats either.

When it comes to stereo equipment, I guess vintage refers to a specific age. Whether or not it matters to you and me depends on the sound.