This is a non-issue in NV- there is a LOT to do in the gameworld without ever completing the 'main' quest. Plenty of sidequests and adventures and exploring to do. And you can do 95% of the main questline right up until the beginning of the final battle sequence, and still keep doing anything else you want to in the game. It's just after you initiate the final battle, that you get a warning message telling you if you continue, you won't be able to go back or keep playing after you do. At that point, you can keep going, or choose 'no', and put it off till later. Or you can do a save at that point, finish the endgame quest, then restart from your save and go about your business. Or, as many people do, finish the main quest, then start a new, mo bettah character and do it all again, and catch all the things you missed on the first run through. It really is no big deal.