At one time Vizio used the code "0000" as a default built in parental control password. If you go to the screen when you setup your parental control password and enter that code you may be able to access the system again and change the password to whatever you want to use.
Unfortunately if the "0000" password does not work you will have to contact Vizio support and have then step you through the process of resetting the password. Due to security issues they will only assist the registered owner with resetting the password. If you go to the website for your TV at the following URL:http://www.vizio.com/e420vo.html#support
On the Top Menu you will see SUPPORT as one of the items you can choose. The drop down menu gives you a number of choices. You will probably have to use the chat option. They will probably ask you to verify the date of purchase on the TV from your receipt. They should then be able to walk you through the procedure to reset the password.