blatantly stolen from the steam forums credit goes to "Racer of all"


"So the problem IS with Steam Cloud (for some reason it syncs the very first save files you made the very first time you played the game) in other words, the save files you make DURING the current session will work until you start up a NEW one.

So first things first, the actual save files are located here, make frequent backups of these files, just in case:
C:\Users\*Username*\Documents\My Games\FalloutNV\Saves\

Now in order for Cloud to not sync back to your old save files and allow the previous ones to show up you have to delete EVERYTHING in this folder:
C:\Program Files (x86)\Steam\userdata\9298493\22380\

(The '9298493' folder might have a different number for you, just find the folder that contains 22380 inside of it, this is the number for New Vegas on Steam). Once that's done, THEN you can launch your game and the previous save files will be there (not the ones you already lost, those are gone, unless you backed them up).

YOU HAVE TO DO THIS EVERY TIME >>BEFORE<< YOU LAUNCH THE GAME, OTHERWISE THE SAVE FILES WILL GET OVERWRITTEN!

Which is why its a good idea to back them up like I said.

I hope this helps, it's annoying, but the only solution until a fix comes. Playing in offline mode MIGHT also work -- but this will prevent you from earning achievements. (simply disabling Cloud from the game does NOT work, I tried)."

Actually one of the first things I did before I even started playing, was disable Steam cloud. I feel much more comfortable managing my saves on my computer. I have had no problems with my saves at all.