Delete your "Baldur's Gate 3" folder under "Larian Studios" in "My Documents", and repair your GoG installation. If you installed mods with loose files, make sure you backtrack and delete the ones you replaced, before repairing. Make sure no mods for BG3 are in Vortex download- or Vortex mod staging-folders.
Then make sure Vortex isn't running, before you start up the game, once. Create a profile and exit the game.
Now start Vortex, and make sure it is configured correctly. Go to Nexus and download a mod with the "Mod manager" button. Install it, and enable it within Vortex. Make sure you accept to Elevate when Vortex asks for it.
Then check the Larian Studios\Baldur's Gate 3\Mods folder and see if any .pak file got installed. (Or any other files that "preview download" in your chosen mod, showed you)
Now boot the game, create a character, and check if it works. (tip; choose a mod that you'll easy can recognize after a few minutes of playtime)
If it works; Install the mods you like, create a new profile (or re-use) and create a new character (important!) and start the game all over again, with your new mod setup.