From Roll20 Wiki
The Character Vault allows you to store your own Characters so that you can use them in multiple games without the need to re-create them from scratch each time. For example, in society-style play you have a single Character that you play many different game sessions with, across different GMs, etc. Previously on Roll20 you would need to keep track of that Character yourself and then re-create it in each new game. However with the Vault feature, you can import the Character out of a game and into your Vault, and then re-export it out of your Vault and into a new game. Rinse and repeat, and now you've got a Character that travels with you between games.
The Vault is also useful for GMs to store pre-generated PCs that they may want to use in multiple Games, a stable of interesting NPCs, etc.
You can import a character to your Vault from any Game you are a player or GM in. However, if you are a player in the game you must have permission to edit the Character to be able to import it to your Vault. Importing Characters is easy -- just click the "Import Character" button on the main Vault page. You'll see a list of Games to choose from. After you choose a Game, you'll see a list of Characters that you have permission to edit. Just choose the Character you want to import and press the "Import Character" button. The process may take a few seconds to complete.
Note that once the Character is in your Vault, it is stored as a "snapshot", meaning that it will no longer update or receive changes from the Game it came from. If you want to import an "updated" version of a Character, just delete the existing Character from your vault, and re-import it from the Game.
When you are ready to use a Character from your Vault in a new game, click the "Export to Game" link next to the Character on the main Vault page. You'll be presented with a list of eligible Games to export the Character to. Note that you can only export Characters to Games where:
1. You are the GM of the Game, or
2. You are a player of the Game and the GM has expressly turned on the "Allow Players to Import Characters" setting on the Game Settings page for that Game.
If you are a free user, the game must also have All Access Vault enabled (see below for more information on Vault Access Levels.)
If a Game you want to export a Character to isn't showing up in the list, be sure that one of those criteria are met, and then reload the page.
Once the Character is exported to the Game, it will be up to the GM of that game to set it up (giving you permission to edit it) properly.
Vault Access Levels
Each game will have one of two access levels, which determines who can export characters into the game:
This is the default level. With this level, only players who are themselves Plus or Pro subscribers (and therefore have "All Access Vault" enabled for their whole account) can export characters into the game.
All Access Vault
This is the premium level of vault access. It allows anyone (including players in the game who don't have a subscription) to export Characters into the game. GMs of the game can also export characters (or NPCs, monsters, etc.) into the game.
The level of a game is determined by the subscription level of the game's creator account. If the game creator does not have a subscription, the game will have the default Limited Vault level. If the game creator has a subscription, the game will have an All Access Vault level.
Additionally, select Marketplace modules (as indicated on their details page) feature the All Access Vault level for all games based on that module. For example, if you purchase Lost Mine of Phandelver, all games you create with that module will have the All Access Vault unlocked, even if the game creator does not have a subscription.