The definition of vanilla software makes it clear that only unmodified software, in this case Minecraft, is considered vanilla. The word vanilla is choosen for this to represent the kind of standard taste (vanilla ice cream is often viewed as basic lmao). Technically, resource packs do not alter the codebase of minecraft and neither do data packs. All they do is provide some data that the game uses to run.

Here is where it gets complicated sometimes though. As you could have probably guessed, using mods is not vanilla. That is, because the code of Minecraft gets extended or injected. It is no longer unmodified. However, data packs can sometimes be structured code-like and can be used to execute functions. This is problematic, because although the source code of Minecraft is still the same, that code could load in other code from some data pack and execute it, essentially giving an effect also achievable by changing Minecraft’s source code. It’s similar with resource packs, although not nearly to the same extend.

So while generally an unmodified piece of software is called “vanilla”, Minecraft itself kind of blends what that means exactly. Minecraft out of the box would be considered vanilla.