It appears that changing the Windows Vista default logon account or removing the auto-login completely is a tricky thing to do.

In this tutorial, you will learn how to enable auto-logon, select a default account for auto-logon, or turn off the auto-logon feature. 

1. Click the Vista 'Start' Icon to display the menu
2. In the Search box type netplwiz and press Enter

3. Choose Continue at the User Account Control confirmation box
4. Place a check in the box next to Users must enter a user name and password to use this computer. 
   1. Checking the box allows you to select a different account for auto-logon.
   2. Or, if you want to disable auto-logon, simply click OK, and you are finished!


5. To change the default auto-logon account, Click on the user's name to highlight the account.
6. Uncheck the box next to Users must enter a user name and password to use this computer.
7. Click Apply
8. In the popup window, enter the password for the selected user account (if it has one) and confirm the password.
9. Click OK