Blizzard said that sharding thecnology was the biggest success in regards to a smoth launch, it is the reason legion launch was arguable the best launch in any mmo ever, it was even weird not getting a queue or not having any kind of lag.
So...Vanilla is gonna be just one server right? so sharding is not possible, so they better build one hell of a server cause nobody wants to experience a laggy launch with long queues for the vanilla relaunch.