I was going to make this a through explanation of what contributes to a vintage tone, however, there really is no point...
TONE IS SUBJECTIVE!
Sorry for the caps, but really... it doesn't matter whether the gear is from the '50s or whether the gear was bought yesterday. What matters is what sounds good to you. To me it seems that the high-gain stuff was not really popular until the '70s and '80s, so the blues, jazz, classical, or whatever was more popular at the time did not need ice-searing highs, massive amounts of distortion, or brutal low-end... It was meant to sound pleasant, sweet, smooth, a bit muddy, magical or whatever sounded good at the time. Now, since the high-gain stuff is quite popular; it's all about definition, clarity, punch, and a TON of distortion because it sounds better that way.
If I had to play metal that sounds pleasant, sweet, smooth, muddy, and magical... then I would sell my guitar and start playing a violin.
But if I had to define what a vintage tone is, then I don't think I could do that. What was 50 or even 25 or even 10 years ago could be considered vintage. I wouldn't be surprised that in the next 25 or 50 years the current sound of metal isn't going to be vintage.
If I had to describe a vintage tone that sounds good to ME, it would be B.B. King. If I had to describe my high-gain tone, I would define it: a brutal tone with enough clarity and vintage flavor combined OR Amon Amarth tone with a vintage flavor, but even Amon Amarth changes their tone with every album.
Anyway, tone is subjective. That is why you have so many knobs on your boutique, vintage, or modern amplifier so you can sound good with your subjective tone, unless you like the tone of your favorite guitar player. That is a different story... still, I know that I wouldn't want to exactly sound like my favorite guitar player and have a little bit of my tone in there, but that's me, not you.