http://www.gamespot.com/articles/fallout-new-vegas-is-getting-a-multiplayer-mod/1100-6434268/

You can travel the lands of Tamriel with friends in The Elder Scrolls Online, but there isn't a multiplayer option for players wanting to roam Obsidian's take on the post-apocalyptic RPG. Modders plan to change that with a multiplayer mod for Fallout: New Vegas, which they're currently trying to find testers for.

New Vegas: Multiplayer will allow you to get thrown out of post-apocalypse casinos with your friends--or attacked and killed by Deathclaws, if that's more your speed (via PC Gamer). Reddit user and modder Nvmp_corporal explains how it works, saying all NPCs in a certain region will be synchronized to a host, but quests won't.

That means that if a player connects to a host that has killed an important quest-centric NPC, then any relevant quest could fail. If the host leaves the region, then a new host is chosen, and the NPCs will resynchronize to them.

Players won't be able to use VATS while using the multiplayer mod, as it's been disabled. If you want to use the inventory, you'll be safe to do so--you become invulnerable when it's open. Items, however, are saved to the server and will get deleted when the mod team wipes the servers during development.