So, essentially, the Known World- later known as Mystara- that was the assumed setting for Moldvay Basic D&D. No extra fluff at all- just the maps with names in the modules. It was the ultimate vanilla setting- because it wasn't a setting- it just WAS.This is a strange question for me. I have never ran nor played in any of these world long enough to get a feel for them. A max of 6 sessions per campaign with no more than 4 or 5 campaigns since I started in 81.
A vanilla setting to me is one with out any back ground/world building. Just the basic assumptions and not attempt to make a full fledged world.
I described Eberron to my group as a D&D Noir-Pulp Mirror Universe. After just two sessions, they said it was a pretty good to explain that world.I would not call Eberron vanilla D&D.
It has all the D&D elements but it twists lots of them. Elves with the Deathless. Halflings ride Dinosaurs. Gods are a step removed and possibly not there. It adds in its own stuff of elemental ships and trains, Dragonmarks, Dragon shards, artificers and warforged, a psionic continent and player race, different cosmology. Its default tone is magical noir post WWI instead of high magic ren faire.
Base D&D is vanilla, Eberron adds on a lot of toppings to make a sundae.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.