The 3 Vaults Location in Slime Rancher
After you've beaten the game and watched the credit roll in Slime Rancher you'll get an email from Hobson telling you about 3 secret vaults that he built years ago to hold his fortune. In order to gain access to these vaults you'll need a Slime Key - so make sure to pop and remaining Gordos if you need to for the remaining Slime Keys.
Inside each vault you will typically find a unique event or puzzle along with over a dozen Treasure Pods. Each Treasure Pod has 10 Gold Plorts in them except the Ash Isle Vault, each Treasure Pod in that vault only has 7 Gold Plorts (or 9). For most of these vaults you're going to need to make two trips because of how many Gold Plorts you'll find.
Depending on your own preferences, I recommend you deposit at least half of the Gold Plorts (if not more) into your Slime Science Refinery incase you ever wish to use them. All of your extra plorts that you plan on selling should be placed into a Silo until the market price for Gold Plorts is above average - then sell all you have.
Ring Island Vault - Dry Reef
You can find the first treasure vault on Ring Island in Dry Reef. To reach this location take the teleporter to the west of your Overgrowth Expansion (a Tabby Gordo sat on it earlier in the game). The entrance that leads into the vault on Ring Island is found a little bit up the mountain and difficult to miss.
Before coming to this location it's recommended you grab some food because we'll be dealing with some Feral Pink Rock Largos. Since they're pink any food source will do, we just want them to no longer be feral. When you're inside the tomb there will be a switch you can hit which will drop the Pink Rock Largos from the ceiling but also open up another section to the vault in the previous room with even more Treasure Pods.
Feral Path Vault - Moss Blanket
This vault is found in the Feral Path portion of the Moss Blanket. You can reach it by either running east from where you found the Hunter Gordo or using your Jetpack and levitating over here from the one way exit portal back to your Ranch. Much like with the previous location I strongly recommend you visit this one with at least 50 of any type of food in your inventory.
In this vault shortly after entering you will come across a set of Pogofruit trees and in the area beyond them about a dozen Gold Slimes. Grab what food you can from the Pogofruit trees then inch forward until you get all the Gold Slimes to spawn. Try not to let them see you while you shoot food at them from a distance; some will walk off the edge and fall into the sea and others will drop plorts into the sea too.
All of this is unavoidable - focus on hitting as many slimes as you can with your food and when you've hit all the Gold Slimes or you've scared them away collect what plorts are available.
Ash Isle Vault - Indigo Quarry
The third and final vault is found on Ash Isle which is the island in Indigo Quarry. In order to reach Ash Isle you'll need to burst Rock Gordo #2 to gain access to the teleporter that brings you here. You'll find the entrance to the vault in the northeastern portion of this island, it's difficult to miss.
Once inside of the vault search behind the single purple Treasure Pod to find a switch which opens up into another massive vault.