Although you may be able to logon locally without a password, Vista just like XP does not allow by default remote access for those accounts. The obvious solution is to set a password for the account, another solution is to change the setting below.
- Run gpedit.msc
- Go to Computer Configuration > Windows Settings > Security Settings > Local Policies > Security Options
- Set Accounts: Limit local accounts use of blank passwords to console login only = Disabled