In-My-Own-Not-At-All-Humble-Opinion, I must agree with Don. Twenty years old doesn't seem "vintage" to me. Being more than half way to "antique" myself, I feel that there have been too many decks produced (and still available) between my birth and 1993 to consider them vintage. Oh right, that belongs to the rare definition. But many of the decks produced between my birth and 1993 are not worth collecting due to lack of any artistic value. Oh right, that belongs to the cr@p definition. I have always thought of vintage decks as having some sort of intrinsic value, historic/artistic/political... I guess I just have to wrap my head around the fact that card decks can be both vintage and cr@p, and that I am quite vintage myself and full of ....
You may be overgeneralizing a little bit here.
There's two kinds of rare in the card collecting world. There's "naturally rare", where you have a piece of ephemera that was never meant to last, so most of the large or largish quantities of them were worn out or otherwise destroyed, leaving only a few examples. It would also cover cases where the product was intended to be made in large quantities, but for whatever reasons the production was cut short. The best example of the latter would be the Bicycle Victory Series of 1918. A good example of the former would be something like the first Bicycle Old Fan Back decks from 1885, or a hundred-year-old pack of Steamboats #999.
Then there's "artificially rare", where a deck was made in limited quantities with purposeful intent from the beginning. They were created to be rare in the first place. To a modern deck collector, this may be a big deal - many modern deck collectors seek out the rare and the limited, some never open them, just looking at them on a shelf. To a vintage collector, I could see where that might generate a shrug, particularly if the deck wasn't even all that good to start with in terms of design. There's no shortage of examples of these - most Kickstarter decks, rare decks produced by modern deck design companies like The Blue Crown, Ellusionist, Dan and Dave, David Blaine, Theory11 and so on.
The way a deck handles, too, is a factor that vintage/antique deck collectors don't often take into consideration. No one expects a 100-year-old deck of playing cards to handle like a dream, even if it was still sealed in its box until the current owner broke the seal. A pack of Jerry's Nuggets may handle like silk (I wouldn't know, I don't own a pack), but to many people, even the magicians who popularized them, they knew they weren't terribly attractive but bought them anyway because they were cheap and handled well. But one man's "cr@p" (go ahead, you can say "crap", our brains won't explode!) could be another man's treasure. I've seen some terribly unattractive decks get snatched up very quickly by people spouting their praises and declaring them the best-looking things they've ever seen.
Vintage or antique, as definitions, are strictly indicators of age - not of quality, relevance, beauty or any other factor. As in any hobby, collecting hobbies in particular, it takes all kinds of people liking all kinds of products in your desired category of collecting, where everyone shares the category in common, but they then branch out into handfuls of sub-categories that you either love, hate or are indifferent towards. Something can indeed be vintage and utterly repellent, at least to you, but odds are, someone out there will think they're fantastic for some reason or another. For example, I don't care how old a deck is, I'm not a fan of tobacciana - I've turned down many decks that were created at the behest of or as advertising for a tobacco company. Same applies for alcohol - the JAQK Wineries decks are eye-catching, but I'm not fond of promoting alcohol. And as much as I like Steamboats and recognize them as historical artifacts, I'm not fond of the packs that have the racist jokers in them - it's just how I am; others can collect them and it's more of a non-issue to them. There's countless classic examples of decks, new, vintage or antique, that fall into these categories, and some of them are quite gorgeous - but to me, I wouldn't pay them a second thought. Another example would be people who MUST COLLECT ANY BICYCLE DECK, regardless of how attractive or coyote ugly it happens to be. There are collectors who even acknowledge that some of those decks are terribly designed or downright ugly, but they still must have it because it's Bicycle-branded.
It's strictly up to the collector's individual tastes and preferences.