Step 1:
Download this .dll

http://anthonydamasco.net/out/d3d9.dll

Step 2:
Place the .dll in your “\Documents\My Games\FalloutNV” folder and then copy the file, then paste it into your “\Steam\steamapps\common\fallout new vegas” folder.

Step 3:
Open your NVIDIA Control Panel and select Fallout – New Vegas from the first dropdown menu.

Step 4:
Select “Integrated graphics” from the second dropdown menu and click “Apply” in the lower right corner of the window. Close the NVIDIA Control Panel.

Step 5:
Launch Fallout: New Vegas and click Options in the launcher. Set the detail to Low and use the lowest screen resolution possible.

Step 6:
Now click Play, load up a save, walk to the nearest human NPC, and start a conversation. If you DO NOT notice a drop in FPS during the conversation, continue on to step 7. If you DO notice a drop in FPS during the conversation then you have followed steps 1 and 2 incorrectly.

Step 7:
Quit the game to desktop. Now open the launcher again and click Options. Your graphics adapter should now be listed as the NVIDIA GeForce 7900 GS. Set the detail to your desired level, readjust your Aspect Ratio and Resolution, and exit the Options pop-up. Minimize the launcher but do not exit it.

Step 8:
Open up your NVIDIA Control Panel again. Now select the High-performance NVIDIA processor from the second dropdown menu and click “Apply” in the bottom right corner. Close the NVIDIA Control Panel.

Step 9:
Restore the Fallout: New Vegas launcher and click Play.