At it’s core, installing mods is roughly the same as any Vortex-enabled game, once you’ve added Into the Breach as a managed game in Vortex.
If you haven’t already, open the Games screen and click the Manage button on the Into the Breach icon under Unmanaged
Remember that mods may still need to be enabled or configured in-game from the Mod Content menu!
If you’re installing mods from Nexus Mods, you can use the “Vortex” or “Download with Mod Manager” buttons on the mod page to automatically open Vortex and download the mod. Simply Install and Enable like usual and you should be good to go.
You can also download mod archives directly from Nexus Mods or anywhere else you like and install them with Vortex. Simply use the Install From File button in the toolbar or drop them onto the drop zone at the bottom of the Vortex window.
Be aware that installing from non-Nexus archives will often not include the full metadata. You can double-click the mod in the mod list to change its details.
Please note that Vortex can’t tell if a mod will be compatible with your current game version and mods may need to be updated to be compatible with certain game updates.