Alright, well, I don't know how to explain this exactly, but I'll do my best.
Currently, I'm running vanilla SP+SMP. I want to try out some mods, however I'm worried that an update might corrupt a world somehow and then I'm all like :SSSS:. So, what I'm interested in doing is creating two separate game files--one for vanilla SP+SMP and my current stuff, and another to try out mods that will be less for putting a lot of time into and more for just messing around with interesting mods. Is this possible?
Yes, but then if I decide to play on my world, wouldn't the mods be on it? And if something later on down the road were to happen that made any of the mods bork up, wouldn't that mess up the world too?
Bah, I suppose I'm worrying over this needlessly. I was hoping to be able to keep vanilla MC separate from modded MC, but I guess I can't.