+1 to what =CB= said.
Its important to make a distinction between vintage and what George Gruhn calls "the golden age" instruments. Just because something is old, it is not necessarily good.
According to Gruhn, "The pieces most sought by collectors are pre-World War II flat top acoustic guitars by Martin, Gibson, the Larson Brothers of Chicago, and Lyon & Healy of Chicago as well as archtop acoustic jazz guitars made through the 1960's by Gibson, Epiphone, D'Angelico, and Stromberg. The golden era for collectible electric guitars was the 1950's through mid 1960's."
From today's perspective, a 1970's guitar seems vintage--but to those of us who experienced them the first time around it is still garbage. So to take a generic term like "vintage guitar" and make a broad judgement that they are over rated, or over valued, or whatever is really missing the point about why people started seeking out certain vintage instruments in the first place.
When I started playing guitar in the sixth grade in 1980 at the dawn of the MTV era, the holy grail of vintage guitars--a 1959 Les Paul Standard--was only 21 years old. To me at the time, 1959 seemed such distant history that it all must have happened in black and white back then I thought. Right after I got out of high school I bought a 1960 Les Paul Junior from a friend. I have now owned that instrument for a longer period of time then the period from 1959 to when I started playing.
My Junior looks, sounds, and plays fantastic and was the smartest $500 I ever spent in my life. I love that guitar and it has given me a lot of pleasure over the years.
But, +1 to what OzziePete said about depreciation and wear and tear. My other hobby is collecting vintage bicycles. Mostly racing bikes from the 40's and 50's, but some others as well. Bike collectors face the same problems as guitar collectors; do I use my vintage piece and enjoy it for its performance and character while I slowly degrade the mechanics with every pedal revolution, or do I keep it polished and waxed behind a glass case for posterity?
Depends on what your intended use is, be it a bicycle or a guitar. If I was going on tour across country I'd definitely bring new stuff, but if I was enjoying the piece on a leisurely weekend afternoon I'd like to get out the old steel and crank 'er up.
So..... +1 with the OP too who says "Guitars now days are better (overall) made AND sound better than they ever have." Overall--for production guitars at least--I agree with this. Our modern technology enables a caring manufacturer to produce a tool to much higher tolerances than in the past. Additionally, since the internet has revolutionized the way we communicate and spread information, there is a lot more attention to niche markets such as "vintage" or "tone freaks" or what have you. When I was a kid a lot of this info was just hearsay passed verbally from one enthusiast to another, so there was a lot of confusion, mis-information, and even some outright scams. But these days there are manufacturers (like Warmoth) who exist solely to satisfy these niche markets.
A lot of it boils down to the manufacturing process itself. Today, there are probably more small custom handmade builders and manufacturers than at any other time in history, so our present era may well be recognized as an additional "golden era" sometime in the future. But by and large the bigger manufacturers all use automated processes to reach their production goals. Although these modern manufacturing techniques allow the tighter tolerances, more consistent finishes etc. that yield wonderful consumer level products, they do lack the personality that comes from a truly handmade work of art.
And that might be the key difference. Instruments made in the "golden era", or bicycles made before about 1964, transcend being a mere tool and due to their individual personalities may well become brilliant works of art in and of themselves.
I'll close with one more quote from George Gruhn, "Collectible instruments may be appreciated as beautiful visual art, important pieces of history, technological marvels, acoustical wonders, and great investments. I can think of no other form of art which can be appreciated on so many different levels."
*George Gruhn quotes lifted from Collecting Vintage Musical Instruments by Gruhn http://www.gruhn.com/articles/collect.html
ps...I will not collect anything unless I can use it, be it guitar, bicycle, fountain pen or watch. I love my vintage wood and steel!