You can install all sorts of weapon upgrade mods in your weapons by going to an upgrade weapons modification vendor in Fallout New Vegas. Following are the weapon modification upgrade vendors you can seek in Mojave Wasteland to install mods in your weapons, installing 20 weapons mods will also unlock mod machine achievement.
Fallout New Vegas Weapons Upgrade Vendors
Great Khan Armorer
You will find Great Khan Armorer in Red Rock Canyon, Northwest Mountains. Upgrade. Guns
You will find Blake in Crimson Caravan Company, New Vegas Conurbation Exterior. Upgrade. Energy Weapons, Guns.
You will find Vendertron in Gun Runners, New Vegas Conurbation Exterior. Upgrade. Energy Weapons, Explosives, Guns.
Gloria Van Graff
You will find Gloria Van Graff in Silver Rush, Free Side, New Vegas Conurbation Interior. Upgrade. Energy Weapons
You will find Dale Barton in The Fort, Northeast Territories. Upgrade. Energy Weapons, Guns.
You will find Chet in Goodsprings, Southwest Desert. Upgrade. Guns
You will find Michelle in 188 Trading Post, Central Mountains. Upgrade. Guns
You will find Knight Torres in Hidden Valley, Central Mountains. Upgrade. Explosives, Guns
Old Lady Gibson
You will find Old Lady Gibson in Gibson Scrap Yard, Central Mountains. Upgrade. Energy Weapons
You will find Cliff Briscoe in Novac, Central Mountains. Upgrade. Energy Weapons, Guns.