I wouldn't call myself a PvP expert, but there are a few steps you can take. Firstly, place your vault at the center of your base, which should be the strongest part. Secondly, surround it with tough materials. Rather than having it out in the open, surround it with walls or foundations to add an extra layer that potential attackers will need to destroy. Finally, throw a fighter thrall or two into the room to guard the vault. If raiders have expended a lot of their resources, a good fighter thrall could potentially cause them to retreat or even kill them, letting you get your loot back.