This guide will help you with the annoying mouse acceleration, so you can enjoy New Vegas without having to deal with it.
Now someone else may have already made a guide on this, but I wanted to post it here just in case.
So recently, I started playing New Vegas, and the mouse acceleration really got to me, so I found a way to fix it.
How to fix it
Go to your New Vegas install directory (this varies, but the default location is in the steamappscommon folder), and find Fallout_default.ini. Open it using notepad, and scroll down to the section titled [Controls].
There, paste the following:
Then, just open the game and enjoy.