The VIGIK system is quite misunderstood overall.
In France, most people live in appartment buildings, so the letter-boxes are not on the street. As such, the post office needs some way to enter the building to deliver mail. Originally, they used a mechanical key, that was universal across the country. The key was copied heavily, which meant that anyone could (and did) get access to buildings.
This mechanical key was replaced by the VIGIK system. "Services", such as the post, power company, etc can get access to buildings using a SERVICE BADGE. The SERVICE BADGES are MIFARE 1K Classic badges.