Diablo 4 (D4) Season 3 rewards players with vault sigils by crafting Pearls of Warding or by exploring Nightmare Vaults, dungeons exclusive to the season. Players are able to farm for gear and aspects in these corrupted dungeons once they reach World Tier 3. Nightmare Vaults are similar to Nightmare Dungeons in that they contain riches.
How to Get Vault Sigils in Diablo 4 (IV) Season 3?

Vault Sigils can be crafted from Jewellers with Pearls of Warding (Arcane Tremors are the source of Pearls of Warding). The completion of Nightmare Vaults (unlocked at World Tier 3) also rewards you with Vault Sigils, just like the completion of Nightmare Dungeons did.
What is Zoltun’s Warding in Diablo 4 (IV) Season 3?

Upon entering Vaults, players receive Zoltun’s Warding, giving them access to the treasures within. The player’s chance of unlocking a Wardwoven Chest that has the chance to drop legendary items will be reduced if they finish the Vault with Zoltun’s Warding intact. If players are hit with traps in the Vaults, Zoltun’s Warding will diminish stack-per-stack.