The best route is Jean Sky Diving -> discover Hidden Valley. Fast travel to Hidden Valley, turn right, run through the gap in the fence towards Black Mountain. Ignore the 2 scorpions and the centaurs, they are idiots and won't successfully connect an attack. At the centaur camp, hog the left wall which leads to a ledge you can use to jump the Black Mountain fence.

Hug the right side once you jumped and continue upwards the mountain until you discover the location. Immediately fast travel to Black Mountain. turn around and walk straight North. You will hit REPCONN HQ and you can choose to go to the 188 for Veronica or take the Eastern road North to Vegas (along the Grub n Gulp).