Easiest way to get Valby Code for unlocking Valby is by simply go to Sterile Land and do the Rockfall Vulgus Outpost to get the Amorphous Material 06. Then go to the Vulgus reactor in the same area and use the material in the reactor.

Simple video with no BS. You do not have to stealth nor use the machines. I simply did it cause its fun and easy as Sharen to do so. You simply destroy the 3 reaxctors in the outpost and the boss will spawn. Kill the boss and you have a 20% chance to get Amorph Mat 06. Simple and easy.