I want to fallout new vegas to be as scary as fallout 3. Is there any mods that makes fallout new vegas scarier??? Besides turning down the brightness.

There are some mods that add extra creatures, but I can't really recommend which is best. I can recommend something like the Rebalance module of Project Nevada that allows you to tweak your stats. You can make it so that a single bullet will kill you and the same goes for your enemies. Nevada Skies comes with a good Night Brightness Adjuster that can make nights quite a bit darker. Other then those I'd recommend going through Vault 34 with no companions, no HUD and brightness turned down.

Define the word scarier.

Harder?
Darker?
Creepier?
Horror-like visuals?

assume him counting as horror atmosphere.

Dunwich building, Fallout 3.

Dark metros, Fallout 3.