I'm with John...I think "classic" has to do with the rounded aluminum models (tt &/or mh) opposed to the painted or boxy ones. Vintage definately refers to 25 years or older...I'll see if I can find out more, and post a clarification if nobody else does.
It says its basically the "classic" airstream shape that binds the members together. They want to ensure we keep "silver bullets" forever. The main activities for the club are caravans. Restoration of Airstreams is not emphasized as much as in the Vintage club.