I haven't found a website that has serial number info, here are a few that may help, though:
(the Vitus company website is only in French, my limited French didn't see any links on it to old stuff.)

http://comteche.us/sitebuilder/users/mavicvitus979/


http://mypage.uniserve.ca/~peterguy/ - they repair Vitus frames and have parts!

http://velospace.org/node/1695 - a picture of my recently purchased Vitus Brevete (1992ish)