The Video Game Championships (VGC) is a yearly tournament, created by The Pokemon Company International in order to give video game players a large-scale ingame competition, similar to what they have already for the Trading Card Game. The rules, however, are very different from the standard metagame we have created here at Smogon.
VGC is not a Smogon tier. VGC refers to the official Pokémon battles, not on Pokémon Showdown. So when people say 'VGC rules' they are talking about the official rules made by the Pokémon company, so there is no Sleep Clause, Evasion clause, there is item clause, etc.
If you want to find out more about the championship, look in the source.