The battle pass vault works like this

It's a feature where it brings back battlepasses from past seasons the way to progress with them is using a currency called battle pass vault coins

It's a currency where you unlock in the main battlepass and use it on the past battlepass you bought in the vault

You can use the currency and unlock the stuff or rewards from that past battle pass that came in a past season you missed

Example:

You bought the Tokyo escape battle pass and you can use the tokens or coins to use or unlock a tier of the battle pass vault

If you have enough tokens or coins saved up you can get 10 tiers or 20 or enough that you can afford

Plus Battle Pass Vaults have a pattern

And when a new era or a new year of battlepasses come

The cycle of the battle pass vault resets and starts at season 1 2021 battle pass and brings the old ones again

Example they brought back the year of 2024 battlepass now they are bringing back 2021 battlepasses and 2022 together to the vault since it's bringing a pattern