Hey guys, I'm making a large total conversion mod for NV and was wondering if it was possible and legal to have my mod launch from a new .exe file? Thanks!

Depends on whats in that exe. If you plan on modifying the game's original exe and distribute that, then the answer is a resounding NO. Modifying the exe is illegal, as well as distributing it.

If you make an exe that launched the games original exe along side it, then that is more probable to be ok.