Without spoiling the ending I was wanting to know whether or not you're allowed to keep playing the game after completing the main quest like in the broken steel DLC for fallout 3. I mostly ask because recently I bought the ultimate edition and wasn't sure whether I should compete the DLC storylines before or after the main quest.

Once it's done, it's done. You will get a message before the end giving you an option to save and warning you though, so pay attention!

You'll definitely want to do the DLC's beforehand, since New Vegas's ending which I won't spoil, It doesn't really allow you to play afterwards. To say anything more would probably be a spoiler.

It might be a minor problem but New Vegas is a great game and you will get a lot of enjoyment out of it.