Vanos was built starting September 1992 manufacture date for the M50 engine. The manufacture date will appear in the driver's side door along with tire specs. There is also a tell tale on top of the engine: the front of it would have a bulge. Along with that, a few upgrades were introduced to the car such as wider mirrors - I don't know much about the other upgrades. I found out about the mirrors when I had a hard time finding replacement glass at a reasonnable price.
Wikipedia entry on Vanos