I'm hoping to keep this civil, but I thought it might be nice to have a philosophical debate about what counts as "vanilla"
For me personally, I find that mods that only add jokers are "vanilla".
My reasoning behind this sort of comes down to my years of playing Dominion, a deck building game, that has 15 expansions. Some of those expansions work well together, as they all share the same base mechanics. Other ones, like Nocturne and Alchemy, add unique mechanics that can work well when added to the base game, or the base game and one other expansion. However, if the mechanics become too diluted by many expansions they end up not being worth pursuing.
An example of this would be a Balatro mod that adds a new type of edition. For example, if a mod adds a Pineapple edition (random example) that mod can have jokers that interact with the Pineapple edition, however other mods won't know you have Pineapple cards and thus this mechanic is diluted. This matters very little if playing with one mod, as that mod can tailor an experience, however if playing with multiple mods this can be a rather prevalent issue.
Meanwhile, """vanilla""" mods that primarily focus on neglected aspects of the base game, for example Power cards, 4 of a kind, Wilds, 7s, are not diluted. I will say though, if the mechanics behind Pineapple editions are cleverly implemented, they can still have interesting reactions with base game jokers and other modded content through dynamic game states.