Fallout 3 has DLC-based post-game content and Fallout 4 has true post-game. But can you play New Vegas (FNV) after beating it?
No, you can not play New Vegas (FNV) after beating it. There is no post-game, and you will be returned prior to the Hoover Dam battle after its completion.
It’s advised to complete side quests and DLC content with your character before “beating” the main game.
The inability to play New Vegas (FNV) after beating it
The climax of Fallout: New Vegas is the battle of Hoover Dam. Regardless of who the Courier sides with, a similar pop-up message appears before the showdown.
A pop-up states the player’s alliance and who they commit themselves with. It says your decision will impact the fate of New Vegas for decades to come.
If there’s anything you want to do before you enter the battle, the pop-up says, do it now. Continuing forward will be “crossing the point of no return.”
The player is then given the choice to say they’re not ready. If selected, the Courier can finish other side quests or further explore the open world. No other parts of the main quest line will be available anymore.
It is likely that a post-game was not included for two reasons. One, Fallout 3 did not have a post game at launch. That was rectified with the Broken Steel DLC, but pressure was not on New Vegas to deliver one.
Second, New Vegas is widely known for being developed during a very short amount of time. Having a fleshed out post-game with your chosen faction populating the world would prove too time consuming.
With this in mind, your best course of action is to 100% the game prior to the final battle. Squeeze as much as you possibly can out of your character before the final confrontation.
The game will prompt you to save before the battle, as to bring you back once it’s completed.
New Vegas’ DLC all takes place, of course, before the battle of Hoover Dam. The player is best off completing all of the DLCs before “beating” New Vegas’ main objective.
So that’s the answer. You can not play FNV after beating it.