I’d say it’s also a bit contextual and on a spectrum.
If you’re using Optifine/Sodium for performance, it’s pretty vanilla. If you get a crash and you use Optifine, you definitely can’t claim to be playing vanilla.
If you’re talking about shaders and you say you prefer the vanilla style, you’re probably talking about flat and pixelated blocks and not the 50 tech mods you’re running.
If you’re talking about how tech mods are too complicated for you, and you like simple vanilla mechanics, you may still be running heavy shaders to make it pretty and you’re probably a builder.