I picked up a nice Vitus the other day and the sun finally came out enough when I wasn't slaving over lab reports to snap a few pictures. I was curious if anyone knows the serial number scheme? I believe mine is mostly period correct Shimano 600 except for the nasty profile stem. I'm looking or a nice Shimano or just Japanese stem if anyone wants to trade for a nice 3T?

I was shocked at how light these suckers are! As pictured, with bottle cage, clip pedals, 105/matrix clincher wheels, large turbo saddle and a computer, it weighs 20.8Lbs. My Cannondale black lightning with full Suntour Sprint weighs over 22...

Also the different stampings on the bike puzzled me. On the head tube 57 is stamped, on the rear non-drive drop out; 58/60. Any ideas what they all mean? I imagine they're sizes but don't directly correspond to any of the measurements other than the seat tube is 57cm c-c.

I know she's dirty and needs tape and a new stem...

Size of frame was cast into the headtube, as you guessed.

Early bonded carbon Peugeots used to use Vitus lugs with an adaptor piece to the carbon tubes (larger than Vitus carbon tubes) but Peugeots were measured c-to-t rather than c-to-c, so the 58 cm Peugeots have 57 cast into the head tube.

Interesting, Any idea what the rear two casting numbers are?

all of the lugs, the bottom bkt shell and the rear dropouts on your vitus are specifically suited for the given frame size.(ie- your rear dropout is specifically made for frames ranging from 58 to 60 cm in size)
on any lugged frame, each lug must correctly & accurately correspond to the required tubing angles, in this case the seatstay angle relative to the chainstay (ex- on a larger frame, the seatstays are set more vertically than on a smaller frameset) .
in order to obtain the best and most reliable join , pressed,bonded & swagged framesets like the vitus must be very tightly toleranced with little room for the fudging of join angles.
on lugged steel framesets using conventional brazing methods, the builder can do some minor adjustment when he mitres and/or files the lugs to fit the tubing (say +/- 1/2 degree or so on premium frames) and in the case of stamped & welded lugs, even bend them slightly to fit. even so ,most high quality(ie-non mass production) frames will require a number of different lugs,btm bkt's and or dropouts of varying join angles in order to complete a full size run of any given model.