I think there will always be a demand for vintage drums, but i do see a slight decline in their interest, and i think it's because of prices..
I've been interested in vintage drums since i was 23, and i'm now 44, so i would think there are still those in their 20's that are still interested, only they can't afford the big bucks these valuable vintage drums demand.. I bought my first vintage kit for $450 Cdn. in '94 during the "grunge" wave, when everyone was buying up thrift store guitars, basses, and drums, because it was cool.. I later sold that kit in '99 for
$800 when the demand began to be significant..(i'm still kicking myself for selling that kit, which was a '76 Ludwig bowling ball black oyster DOH) Ignorance is also a factor with the younger generation, because they more often assume that because a certain drum kit was made in the 60's, 70's, or 80's, and that it's dingy, dirty, rusty, or in bad shape, that they're cheap or valueless..
I think another factor is the custom U.S drum companies that are making vintage style drums, such as C&C .. A young cat may figure that he or she may as well buy a new kit at the same price that they would pay for a vintage one, and get a brand new, vintage style drum kit, without the hassle of flimsy or hard to come by hardware,rusty, broken or missing parts, etc.
In any case, the vintage drum market is just like any other market, in that there will always be fluctuations.. I own 3 vintage kits, and one modern one, and i do get the urge from time to time to buy something from the last decade or so, and maybe one day, i will. But if i do, you can bet that it wll be modeled after the drums of the past :p