Where is the latest I can Start Dead Money from?

My main saves are in the Legates Camp but I always seem to get moved into end game and get the final credits, even if I have got the 5 level extension and set my mission as Dead Money

If you're at the Legate's Camp then you've already begun the final mission, making it impossible to use that save for Dead Money. Other than that you can start it anytime in the game.

If you have the PC version you will be able to use console commands or mods to get out. But if you are on a ps3 or xbox then you are out of luck.

Remember that pop-up box you clikced yes on and then got teleported to the dam?

Well by clikcing on that box you cannot get back to the rest of the game. You have to revert to a save before that.....if you cant, you'll have to start a new character.