Let me give you an example from another world I know well: fishing tackle. Some time in the 1970s when graphite was becoming the fishing rod material of choice, some writers of fishing books and articles in fishing magazines began writing discourses about the wonders of "split cane" and the classic makers of bamboo fly rods in the old days, and how wonderful they were. You can guess what happened next. The value of old bamboo fly rods shot through the roof. This was all well and good as it related to rods by the master makers but all of a sudden bamboo fly rods that were better suited for tomato stakes were being sold for silly money. What people seemed to forget was that, before about the mid 1950s, ALL fly rods were made out of bamboo, and that included the no-big-deal mass produced rods that were sold at the local hardware store for a few dollars. They were barely usable then, and they were no better (and quite possibly worse!) 30 or so years later. Eventually people came to their senses.
Today, I see the same thing happening with guitars. Not surprisingly, the place where it most absurd and laughable is on Craigslist. Who knows? Perhaps those "vintage" Stellas that were sold sold by Sears back in the 50s and 60s really are being sold at their listed prices of ten or twenty times what they sold for when they were new, but dear Lord, I hope not! Those things were almost unplayable plywood pieces of junk back then and they are now. They usually featured fret edges that sliced up the inside of your hand, action that made playing beyond the third fret out of the question, intonation that made even a perfectly fingered chord sound wrong, tuners that barely turned or wouldn't hold if they could be, but most of all a sound that was little better than a cigar box with rubber bands strung across it. But heck, they were cheap and many of us (myself included) made our first attempts at a G chord on one. But lets get this straight, folks, they were junk then and they are now.
Likewise, many of the strange looking electric guitars that were coming out of Japan and strangely, Italy. Featuring a dazzling array of buttons and switches that may or may not have done anything and finishes often referred to as "Mother of Toilet Seat," they all had two things in common: their pickups were guaranteed to generate horrible, screeching feedback at anything over the lowest volume - and compared to even the least expensive Fender, they were cheap. Today there are entire web sites devoted to the dozens of bizarre instruments of this genre but I'm hard pressed to believe anyone ever really plays them in anything remotely like a serious or professional setting. Of course, if you have one on a boat with you and the motor breaks down, they make excellent paddles. But these things too will be found on Craigslist and elsewhere for astounding (asking) prices. Why? Because they are "vintage" !!
However - and I know I'm repeating myself - an old Martin or Gibson or pre 1960 Epiphone can rightfully called vintage. Here's where it gets sticky though. Does vintage mean "better?" That is so totally subjective I wouldn't begin to hazard a guess. I do know that a vintage American-made guitar will be more valuable than a recent one, regardless of how it sounds. Just how much more valuable depends on many, many things. Surprisingly, overall condition is not quite as important as one might think. I've seen Martins and Gibsons from the 1950s that looked like they'd been through a war get sold for astronomical prices. The better the condition, the better the price of course but even things like cracks and worn away finish don't have a huge effect, especially on guitars that are particularly desirable like anything made with Brazilian rosewood. Not unlike valuable antique furniture, old guitars that have their original finish and parts will bring the best money. Models that were only made in limited quantity really up the ante too.
The rub for me is one of a practical nature. I don't have the luxury of buying guitars purely for investment. I wish I did but I just don't, so I have factor in both investment but much more importantly, sound and playability. The fact is that we may be in another "golden age" of guitar building right now. The average guitar coming from all the American companies and a few in Japan in many cases sounds just as good as some of the middle-of-the-line vintage instruments that are bringing huge dollars. What is that vintage guitar really worth? What someone is willing to spend, I guess. Don't be fooled by the so-called Blue Book of guitar values. The numbers in there are extremely arbitrary, not unlike those catalogs that put value on baseball cards. You can put whatever price you want on something. The market will tell you if its worth it or not. End of story.
But just this in closing. Please, please, please don't fall into the trap of assuming some guitar that's 40 or 50 years old is "vintage" just because it's managed to last that long. A $20 piece of junk in 1965 is a $20 piece of junk today, Or perhaps a canoe paddle.