You would need to use the email that you did when you signed up for the trial
Can you give more details on 'how' you logged on to your my.avast account ?
You should be able to do it from the avast tray icon, Registration information, see attached image. Or you should be able to connect to my.avast.com directly, if you are logged on to the forum it use your logon information you used to connect.
If the VPN was free 7 day trial without supporting payment method, uninstalling it 'should' be enough, but as an Avast user not an avast team member I can't guarantee that.