The first hidden vendor is pretty simple to get to, first you must fast travel to the Muir Processing Plant and drive to the top of Crystal Pond (as shown below)



When you arrive you will need to kill some 99ers before the merchant will become available

This one is also relatively easy to get to, you fast travel to Top Notch Toolworks and drive to the merchant located below



Once you arrive you will be greeted by 99ers just like the first vendor, kill them to get the vendor to appear

The Third Vendor is the easiest to get to, you fast travel to Top Notch Toolworks and go to Iron Demon Ranch, you can see this on the map as the Minigun Mayhem Rampage mission is there or follow the map



Kill the 99ers just like the other two and buy your items or play the rampage mission