well i'm definitely not an authority, but when i got into bikes and was interested in fixing em up, my "bike mentor" as i called him told me that any steel bike with downtube shifters that was from the bike boom or before is what made it a vintage classic. i'm sure many would disagree with that definition, but basically any old bike is what i took away from that.
so, according to him (and now me!) that means no crabon or alum, no disk brakes, downtube friction shifters, anything older than the 1990's. so, thatsa lotta bikes.
and now i'm ready to get flamed for this post!!
edit just to add that perhaps some aluminum frames could fall into this category. and also, new bikes that fit the mold are (ime) typically referred to as neo-retro). but whatever, they're all just bikes