Sellers market forold bmx's?
Great! I got 4 in my flat here! Anyone want a Diamond Back Silver Streak?
Seriously though, there have been lots of frames through the years, but few if any MTB's are "clasic" as there has been a lot of development and changes through the years, and fashion influences too.
IE my first Clockwork Orange was a lovely bike, best frame of any of the bikes I've owned since. But with its canti bosses, and only suitable for suspension up to 60 mm, and it's 1 inch head tube, it would be a no brainer to buy second hand today, as you couldnt find forks to fit it! The same will be just as applicible for the Orange Vit ti frames, ie fork length and the fashion for longer, means it will only sell to a small retro market.
Old racing frames are a little different as single speed makes a comeback with all these fixies and the rlj's who ride them but your frame will sure as tomorrow is friday, not retain value.
Buy it for you, but dont justify it as an "investment". Even my DB Silverstreak as mentioned above, is only worth today a bit over what I paid for it 25 years ago, and taking into account inflation, thats stilla thumping loss, despite that I could go to RADBMX and sell it 10x over, as people love them! Perhaps a Harry LEary Turbo in mint original condition might buck the trend, I've seen a couple go for £1k+ and one for £2k that was virtually mint original down to its irc skinwall comp II DB specific tyres.