Fallout: New Vegas weapon mods

Overview
One of the features added to New Vegas is the ability to custom modify certain base weapons. While in Fallout 3 this was limited to the creation of custom weapons, New Vegas takes this a step further by allowing the creation of weapon variants based on vanilla weapons by adding weapon mods.

Unique weapons cannot be equipped with weapon mods. While unique guns usually have better base stats, Obsidian Entertainment introduced this inherent drawback of unique weapons to balance out unique and normal weapons and make the choice less of an obvious decision and more of a preference.

The weapon mods by themselves serve no function, but once attached to their specified weapon will change the characteristics, look or function to create a distinct variant. The weapon mods range from scopes and silencers to extended mags and several in-between. In the case of the Varmint rifle and other such weapons there exists more than one different type of modification. While you are not limited to the amount of mods you can apply to a single weapon, you are limited to one of each type of mod per single weapon and therefore are not able to stack attributes like increase rate of fire.

Once a weapon mod is attached to a weapon it can not be removed and remains a permanent feature of that weapon, along with its added attribute. Also a modded weapon may no longer be used to repair base versions of the weapon: this maybe a bug or a glitch for some users, so be wise in what you want to attach. For example while an extended magazine is almost always beneficial, a scope on the other hand has pros and cons. It may increase maximum range, but makes it harder in close combat situations.

Weapon mods are attached via the PipBoy, to attach a weapon mod you must have both the weapon it is intended for and the mod in your inventory. Highlight the weapon you want to mod within the items section of you PipBoy, then selecting the modding menu via the button as indicated in the top right of the info panel. After you have selected a mod, the weapon name will have a + after it to show it has been improved.

Attaching 20 mods to weapons grants the Mod Machine trophy/achievement.

Vendors
There are currently 11 vendors who offer weapon mods (inventory of mods changes with stock rotation) in the Mojave Desert:

Killing the vendor doesn't allow you to pick up the weapon mods from their inventory.
 * Alexander, 188 Trading Post (Need at least 40 guns skill to pass challenge)
 * Blake, Crimson Caravan Camp
 * Chet, Goodsprings
 * Cliff Briscoe, Novac
 * Dale Barton, The Fort
 * Gloria Van Graff, Silver Rush, Freeside
 * Knight Torres, Hidden Valley
 * Michelle, 188 Trading Post
 * Mick, Mick and Ralph's, Freeside
 * Old Lady Gibson, Gibson Scrap Yard
 * Vendortron, Gun Runners