The official rule list for VGC 2017 has been released on the official site. This confirms much of what has been suspected through Battle Spot but includes a few new tidbits. Below is the list of the rules:

• Usable Games Pokémon Sun, Pokémon Moon
• Battle Type: Double Battle
• Pokémon Restrictions: Alola Pokédex only
• Banned Pokémon: Cosmog, Cosmoem, Solgaleo, Lunala, Necrozma, Magearna, Pokémon not captured or hatched in Sun & Moon
• Banned Items: Mega Stones
• Clauses: No same Pokémon, No same item
• Players may only use Pokémon with items or moves obtained in Pokémon Sun & Moon. Any moves unable to be obtained without Pokémon only available in the generation through Pokémon Bank are banned until Pokémon Bank opens up
• It also institutes a 50 minute timer for the overall match in best-of-three matches. This timer lasts for 50 minutes and all three games are to be completed in this time. If the time elapses and both players have won 1 game, then a Sudden Death game begins where players have three turns to gain a significant advantage and, afterwards, the winner is decided. If no clear winner is visible, then the round will continue until an advantage is seen

(All information here is taken from http://www.serebii.net/ and is used for informative purposes only.)