I was wondering which rifle, mod or vanilla, would allow me to kill from the furthest distance. I know in real life, some rifles allow shots from around 2 miles or so, possibly more. I know that's probably not the case in FNV, but the thought of being able to kill a fiend or ghoul from a safe distance sounds wonderful. As of now, what rifle has the longest range?

Try the anti-materiel rifle (longest range in VATS and probably the best long range rifle, Spread: 0.015) or Paciencia (unique hunting rifle [non unique is also good], Spread: 0.01).

Technically they all have the same range, being hitscan weapons.

The ones with lowest spread and good sights are going to make it much easier to hit targets far away though and the AMR with match rounds is the best at doing this.

Are those vanilla weapons or mods? Where can I find them?

This is always a good place to start if you want information about any Fallout game:

http://fallout.wikia.com/wiki/Anti-materiel_rifle_%28GRA%29

http://fallout.wikia.com/wiki/Paciencia_%28GRA%29

There is also the Vault: http://fallout.gamepedia.com/Fallout_Wiki

They are nearly the same, but I like Nukapedia more.

For vanilla weapons, the Sniper Rifle, its unique version the Gobi Campaign scout rifle, a fully upgraded Hunting Rifle, and Anti-Materiel Rifle are probably your best bet. With mods included, you can also look into Paciencia, the GRA Anti-Materiel Rifle, and Christine's COS silencer rifle.

And if you'd rather go for energy weapons, the vanilla game has the Gauss Rifle and its unique counterpart the YCS/186, as well as the fully upgraded Laser Rifle.