To remove a user from a Vault, go to Vaults by pressing the shield icon in the column on the left  

To remove a user from the Vault, press the bin icon next to their name in the Vault.

You may want to change the owner of the Vault as they are responsible for adding and removing users. As the owner, you can edit the owner by pressing the pen icon next to your name, then the cross next to your name, search for another user and "Assign Owner". 

You can add users by pressing the plus button in the Vault heading and search for users in Lumi to add.