actually the best way to stop the game from patching automatically is by:
Steam Library>Right Click name of game>Updates Tab>Do not automatically update this game.
Steam forgets that setting when you exit it.
I think it switches back if you go online.
I, however, was offline and had Do Not Update checked. I needed to go online to check NVSE's requirement that Steam Community be active (I have never had a satisfactory explanation as to why this must be so.) Strangely, Steam Community can be active even if Steam is "offline."
When I tried to switch to "Online" mode Steam crashed. I could not restart it because the Steam32 process was still running. I suspect it was downloading in the background. By the time I got it shutdown and restarted the deed was done. When I started NVSE I got the dreaded "Newer version of Fallout than NVSE supports" dialog. and it won't run.
YAY!
Here is the official statement from the NVSE page at Silverlock:Note: Bethesda has released a patch for Fallout: New Vegas (1.2.0.352). I am working on updating NVSE to be compatible with the new patch.