I tried this , this morning and noticed a little less lag with people around

Update to newest driver set for your card (260.89 for 9600 GT my case)

In the Nvidia control panel
Manage 3D settings
Program settings Tab
Add New Vegas program to the list
Power Management Mode change from "Adaptive" to "Prefer Max performance"
Save

Run game from NVlauncher.exe

It will do the little steam popup [Starting Fallout New Vegas]
but it seems better.

like most gamesas engine might not work for everyone , but worth an try.

I've seen this mentioned a few times elsewhere, my problem is that Power Management options are not visible under my 3d management section.
I went to nvidia's site to see if my card was compatible (9600gt 512mb) and it lists all the 9 series as compatible.
Not sure what the deal is but that option is just not there. I am running the latest drivers as well.