Yes, you can return to the Vaults you have been to before to restock since any items (including Decaying Eldarium) will respawn after a while. The Vaults loot chests and enemies will keep having the same type of items available for you the next time you loot them again (depending on the items respawn chances). However, you'll need to wait until the game respawns new enemies and chests items by staying away from the location and returning there later. Once you leave a Vault you're planning to return to and get more Decaying Eldarium, you must wait for that Vault to reset so you can enter it again. Upon completing a Vault and leaving it, the entrance gets locked for 15 to 115 minutes, depending on the server or session you're currently playing in. If you haven't gone through a Vault recently and it is still locked, it's because another player has completed it recently, and you'll need to wait until it opens again.