Signing in to your account will work a little differently once you enable your Two-Factor Authentication.
1. Enter your username and Password as usual and click "Sign In". You will receive a code on your mobile.
2. Enter the code from your phone in the box "Phone PIN" and click "Enter".
If you always use your personal computer or a particular computer to login into your Vault account, you can click on "Trust this computer" and you will not receive a new code. That computer will be recognized as one of your trusted computers.