When you first play through the rogue-lite shooter Gunfrie Reborn you may notice some glowing purple cracks on the wall. There are potential portals that can lead you to vaults. When you try to interact with them, you will be told that you need a skill to use them.
The skill in question can only be unlocked outside of an active run, and you will need a resource called Soul Essence to do it. Soul Essence drops from enemies, and it can be used during a run to revive yourself if you die. It is also used to unlocked Talents for your character, so make sure you leave some free at the end of each run to buy more Talents.
After you finish a run, hit the Talents tab, then look at the Expedition tree. You will need to unlock the first skill, Exploration, to be able to use the Vaults. Now, on your next run, you will be able to interact with the cracks to open the portal, then use it to be dropped into a side dungeon.
The smaller vault dungeon will not change the level you are in, and when you exit you will be placed at the original Vault portal. Kill any enemies you find inside, then track down the chest that will be waiting for you with some extra loot.
Sometimes you may need to damage the wall to fully expose the Vault Portal, explosives of any kind are the best option for this. You should always try and explore Vaults, as they will all end with a chest, which can give you power items, schools, weapons, or skill variants for your run.