This is what I was getting at.

You'd need both games to play such a mod, but there's no reason why telling the new GECK to look in the old folders wouldn't work.
I mean, as long as you don't distribute Bethesda assets, there's nothing illegal or against the EULA being done.
Just for starters. . .
Chems are diferent in the game
Weapons are diferent in this game (the sniper rifle and silencer are now mods you apply to a weapon for just one thing, thats not counting the change to weapon degridation and repair)
you would need to code in a way to give an actual scoped weapon to the handfull of snipers
probably even the faction reactions and how the game handles it is completly diferent.
Heck even the skillbooks are probably diferent. Saftest thing would be to replace each skillbook with a comprable skill magazine.
Probably have to do the same with weapons, replace all existing references with the new Vegas weapons