A lot of people ITT are assuming these servers are just going to be rolled back to another patch and that its all "Nostalgia" and "going to suck" but the reality is they are most likely bringing out a hybrid with content/restrictions from vanilla but specific QoL features from the current game. That's why they specifically say "vanilla experience" in their announcement.
Are they are going to slowly release raid unlocks which means perhaps another AQ event? The questions are really endless with this announcement.