RE: VNC, Screen Sharing...
Hi,
To be honest, I've never tried connecting via VNC/Screen Sharing without using some form of authentication, so I can't answer that question (and I don't want to test it since I don't want to create an account without a password).
But a solution to your problem would be to create an account with password, and then allow Remote Management to your family Mac for this new password account (and not allow Remote Management to your parent's password-free account). You can Screen Share even when someone else is logged into the family Mac...meaning your parental units can be logged into their password-free account, and you can Screen Share from the Internet by logging into your password account. When you do this you will be given the option of Sharing their screen or starting a new windowserver for your VNC session. This means that you can share their screen to help them do something, or you can have your own GUI screen for performing administration tasks even when your parents are logged in to their account -- they won't even know you are also logged in.
So, as I said, I have never attempted to do VNC without a password, so I don't know how this works. On the other hand, I do VNC all the time from Macs to Macs, from Macs to Linux boxes, and from Linux boxes to Macs, and all of this works without problems.
Good luck,
Switon
P.S. I'd still try to get your folks to use a password, especially when you leave. If their machine is compromised, you won't be around to figure this out for them.