In FO3 you could pretty much go wander the wastes as long as you did'nt enter the monument during the last mission but when you did you were pretty much stuck with finishing the game. (until Broken Steel came out)

So since FONV probaly won't have that sort of DLC if you pass a certain point during a mission you'll be trap there until you finish the mission and end the game but when is this point? I'm helping the NCR and it sounds like theres gonna be a huge war bewteen them and the Legion soon.

I want to know at what point will i not be able to freely wander the wastes and finish up side quests because after putting as much time into my char as i have i don't wanna overwrite all my saves at a point where i have no choice but to finish the game.

You will be presented with a pretty clear dialogue box directly before you can no longer just go do whatever your heart desires. Not to spoil the ending, but it's after you deal with (in a good or bad way) Mr. House's line of quests.