Viva New Vegas using Wabbajack and Flatpak Steam.

1. Use Flatseal and add a directory to Steam (for example ~/Documents). This is where you will save Viva New Vegas mods and Wabbajack.
2. Install Wabbajack in ~/Documents/Wabbajack using this tutorial: https://github.com/Omni-guides/Wabbajack-Modlist-Linux/wiki/Wabbajack-via-Proton Download Viva new Vegas from Wabbajack and point it to ~/Documents/Viva.
3. Follow this tutorial to install Steam Tinker Launch for Flatpak https://github.com/sonic2kk/steamtinkerlaunch/wiki/Installation#steam-flatpak.
4. You should restart Steam to have Steam Tinker Launch available.
5. Change the Proton version used for Viva New Vegas to Steam Tinker Launch.
6. Play the game once then follow this guide https://github.com/Omni-guides/Wabbajack-Modlist-Linux/wiki/Fallout-NV:-Begin-Again#step-8---switch-to-steamtinkerlaunch-and-configure-it.
7. Mod Organizer should launch, please follow the official guide and setup everything else.

Thanks to https://moddinglinked.com/. Hang out in their discord for some help.