The Vampire Job is very unique and complicated one to get. It is also very powerful. I will tell you how to start the quest and guide you through each location you must visit before opening the Vampire Castle. If you want a guide for using the Vampire Job after unlocking it, check out my Vampire Job Guide.
This guide will cover the Vampire Quest line including:
Vampire Castle: Where it is, and how to get there.
Keystone Locations: Where to find all 6 keystones.
Elemental Dragons: How to defeat the dragons at each keystone.
Vampire Castle Revisited: Defeating Lord DeRosso and gaining the Vampire Job.
You can reach Vampire Castle in Chapter 4.
After defeating Braev the Templar, go to the south exit of Eternian Central Command, towards Eternia City.
Take the path to the west and defeat the zombie dragon if you haven't already.
The dragon isn't that hard, but you have to kill it twice because it will revive itself.
Follow the path to the castle, it is the blue marked location on your map to the northwest.
[Cut-scene] After watching the scenes, you can now get the 6 keystones.