Nit shall be picked:
In violation of contract/EULA != illegal.
That said, if the assets are already included (likely are, look for info http://www.gamesas.com/index.php?/topic/1123578-modding-asset-anolysis-for-fo3-and-fnv/ then the point is moot.
Then let me return the favor.
Unless specifically said, transferring assets from one game to another is illegal, as it breaches copyright laws.The EULA says you can not share assets between games, be them Bethesda IPs or any other, therefore, using assets from Fallout 3 in Fallout NV would be illegal.
That being said, it has been said that shared assets between Fallout 3 and NV are free to use between games. I did not know the stealth suit was in NV, but I do now, so I retract my statement about it being illegal.