The Lexis Rex website contains free games and exercises to give you a variety of ways to keep your Vietnamese vocabulary building, but you may want to use the site free of advertising.
Access to the ad free site is $8 USD for one year. Your User Id (shown here or via the yellow Settings menu option) will be used to identify you, without the need for log-ins and passwords. Once you have paid via PayPal you will automatically have access to the ad free version after a few minutes while the payment is processed. You will receive an acknowledgement email along with a copy of your User Id.
You may prefer to become a Member instead, this also removes the ads but comes with additional benefits, click here to find out more.