I have been trying to fix some performance issues that I have been having with Fallout New Vegas with the same tweak guide I used for Fallout 3 and fixed everything up how I fixed Fallout 3 and when I saved it complained about it being read only so i saved under a different name. Made Fallout.ini not read only and then saved my version over. Started up the game hoping my changes would take affect. Notably slight FOV tweak to make pip boy not in your face, and as not expected the game replaced my file. I do not have steam cloud enabled on Fallout New Vegas because of the save bugs and see no reason for the replace. Ran Steam offline and STILL replaces my file. What is going on with this, I need to perform these edits or else I lag out due to low game cache.
Bethesda if you are reading please fix this issue in an update, I just cant play the game like this.

R-Click > Properties > Untick "Read only" flag (for those who haven't gottten that far)

Also, in your FalloutPrefs.ini (not read only by default)...

fPipboy1stPersonFOV=55.0

...should pull the pip away a tad and the change seems to stick.

Fallout.ini seems to be uneditable. The file seems to get replaced whenever NV is launched. (Or maybe just when you edit your options in the launcher?) I tried to put the mouse acceleration fix in there, but my added lines were gone after launching the game.

You can edit FalloutPrefs.ini in \Documents\My Games\FalloutNV, or Fallout_default.ini in \Steam\SteamApps\common\fallout new vegas.

I usually uncheck read only in the properties of each and do my ini tweaks and then recheck read only and my settings seem to be sticking. I have noticed some of the Fallout 3 stuff that needed to be in Fallout.ini now need to be in FalloutPrefs.ini like making ArchiveInvalidationInvalidated! work for New Vegas texture mods. Overall Steam or Obsidian seems to have made more problems that were not present in Fallout 3.

I'm not a fan of this change either!

I've been doing things one way on this same engine for years, now I have to learn something new, on the same craptastic engine. Grrr.

I'll live though.