I want to play vanilla Fallout: New Vegas, but it keeps installing with mods that i don't know how to remove. I assume this has to do with cloud saves of the game. How do I stop this from happening?

Fallout: New Vegas Ultimate Edition includes all DLC in a single installer package. You can try something like FOMM fork or maybe the Vortex mod manager to disable that DLC. I recommend just ignoring the content you don't want instead of potentially crippling your installation.

As far as I can remember all DLCs for games like Fallout 3 or New Vegas came as separate packages. You can use any mod manager to deactivate them and also delete if you want to free some additional space.

I couldn't disable the mods with any mod managers, but i found a different solution. There's an executable in the game folders, called unins000, and if you use that and then reinstall the game, it uninstalls the mods.