If you have a cracked version of FNV and want to play with mods, the fnv4gb launcher is a must. Problem is, it won't work on cracked .exe's of the game. This fixes that, allowing you to use the fnv4gb.exe with the cracked exe. Just follow the instructions on here or in the readme.
Thanks to metabuddy on Kickass Torrents for sharing this fix. I couldn't download his torrents due to 0 seeders (lol) so i decided to reupload it. All credits go to him (or her).
1. Copy either 32 or 64 bit version to your Fallout: New Vegas directory (where FalloutNV.exe is located)
2. Drag and drop FalloutNV.exe over PEChecksum.exe, if windows asks, allow it to modify the file
3. Drag and drop FalloutNVLauncher.exe over PEChecksum, just like step 3.
4. That's it! Now you should be able to launch the game using FNV4gb.exe (4gb patch) and no longer get a steam error
Don't forget to seed. :)

Cracked FNV4GB fix