Vanilla+ is a lie.
Any addition added to Vanilla makes it more and more not Vanilla. Nobody can even agree on what constitutes Vanilla. Does adding an LFG tool work for a Vanilla+ server or does it make it too retail? What about summoning stones? There's no consensus.
But in truth Vanilla no longer exists. It died years ago. Because True Vanilla was not a decades old solved game. You played Vanilla not knowing what mysteries you would discover. The best times I had in the game were finding something weird just on my own. That happening again in the same game I've been playing for almost twenty year is not happening. Even if it's your first time playing you are playing with others who have a solved game mindset. So even a perfectly replicated vanilla server isn't really vanilla.
The truest vanilla server would be a completely custom server where it's New Game+ and all the quests and mobs and dungeons and raids and items and maps are different and even some of the lore is different. New classes. New races. Then you have a vanilla-like experience of discovery and wonder even though it's not vanilla.
Ultimately servers like Turtle Wow and Epoch should focus on providing players with a new experience that is fun and new, and not worrying "is this vanilla" so much.
Personally I would say the server that may come closest to providing anything close to a vanilla-like experience in terms of rolling back the clock and providing something new for you to adventure in and discover and explore is maybe Conquest of Azeroth.