|The PLEX Vault is a separate inventory that only you have access to. It allows for safe storage of PLEX while moving as well as facilitating selling PLEX, adding Omega game time (plexing your account), Multiple Character Training (MCT) as well as buying PLEX.|
Safe storage of PLEX
Items in the PLEX vault will not drop or be destroyed if someone kills your ship and pods you, so it's a safe way to move your PLEX.
Since you need to have your PLEX in the PLEX vault to use it or sell it (it won't let you sell PLEX if it's in your hangar or cargo hold), there's no reason to take it out of the vault unless you're planning on giving it away to someone through direct trade or contracts.
Adding physical PLEX into the vault
If you've purchased PLEX from the market or received a physical bundle of PLEX from someone, you can add it to the PLEX vault locally as well as remotely.
PLEX vault services
In the PLEX vault window you have quick access to selling PLEX directly on the market, adding Omega game time, buying extra skill queues, shop in the New Eden Store as well as buying PLEX using real money. To open the PLEX vault, go to your unified inventory (either from your shortcut NeoCom), it's located at the very bottom of the index-list.+ or the
Sell PLEX for ISK
This option will simply open a normal Sell Items window. It won't open the market window however, so you might want to do that manually before selling the PLEX, so you can see the current prices and trends before setting your own price.
Add Omega game time & Buy extra skill queue
Both of these options open up a mini-version of the New Eden Store for quick access to these services and items.
Shop in the New Eden Store
This option will require fast checkout, but will conveniently allow you to complete PLEX purchase for real money without leaving the client.