The treasury is mainly a pay-to-play feature, that the free-to-play players can unlock from time to time as well. The rewards inside it are based on how much you've spent in the past, therefore, if you've spent more real money, the rewards inside are better. If you've spent less in the past, then you get less.
To open the treasury, you have a period of time monthly where it is full and can be unlocked. To get to this point, you have to do your dailies and collect your daily chest, or kill hostiles. Each daily is 100 points, each hostile is 1. You will almost certainly fill it without thinking about it, so no point in grinding this.
Then you need a Desealing Rod. You can either get this from the pay store for ~100 dollars, or you can wait and collect them from events.
The easiest way to get it a desealing rod is to pay real money for it. It shows up in the store as soon as you've unlocked the vault and the 700 points you need to score.
You can collect Broken Desealing Rods from two places usually. One is in your 30 Day Loyalty Chest. The other is in Heroic BP Overflow events (even from the small officer ones), where once you've completed the Battlepass if you complete the rest of the daily events you get another reward. You need 4000 of these to unlock the vault, so it's effectively every six months you should be able to unlock this.
Once you've collected all these fragments, you can buy it. It shows up in the recruit section as soon as you've unlocked the vault and the 700 points you need to score.
