It has been considered many times before and in the end its just too much work to be worth it. On top of the hundreds more meshes that would need to be edited for FNV compared to the 40 or so in F3, a load of animations would need to be made from scratch. There's also the matter that editing the weapon nifs to have the right positioning in 1st person for the iron sights would leave many of them looking goofy in 3rd person.
Xilandro has come up with a proper solution for the ugly iron sights transition, I'd just wait for that.