To me vintage refers to wine of a certain year, or more generally a thing of a certain age, but that age isn't implied, it has to be specified. There's no implication of quality. A vintage (=aged to some unknown extent) violin with a cardboard top isn't going to sound very good. A 1,000-year-old book's pages will, barring miraculous preservation, disintegrate the first time I try to turn them. But it's totally fair to call it a vintage book.

I wonder if the assumption that age=quality spills over from the assumption that the only good red wine is old red wine. This assumption is probably rooted in the fact that white wines don't generally age as well. Which has what to do with instruments? But yes probably people make assumptions when they hear the word "vintage" as they do when they hear of "lite" food and "diet" drinks. There is no useful meaning in those words, that's why they are used.

I think the problem of implied quality borne of age is a matter of functional illiteracy--a lot of people know what the individual words on the screen mean but do not know what the whole sentence or paragraph or document means. Those people are necessarily unable to think critically, to read between the lines and make an educated guess what may be deliberately excluded or duplicitously included. At the same time, I'm not sure everyone who writes the ads has enough functional literacy to know that they may be misleading the public; I'm sure some do but I expect many don't and simply use words they've heard others use without ever bothering to check what they actually mean.