Unlike a tradition steel frame where there is the builders name and a separate tube makers decal, Vitus was both the builder of the frame and manufactured the tube set. So the small tubing decal also reflected the model number, and it was really just there to give the bike more of a traditional look. All of the frames were made with the same Futural aluminum. It was the "FUTURe of ALuminum". At least until the mid 90's and Cannondale showed up.

As for the numbering scheme, who knows. They were French. But the last two digits were the year the frame first hit the market. I always thought the 992 should have been the 292 in keeping with the palindrome like scheme. At least they stopped making French and Swiss threaded frames by the early 90's. It was a real pain dealing with those bikes.