With the Apple Screen Sharing based VNC plugin you can request permissions from a remotely logged on user on another Mac to view/control his screen.
To do this, either set the credentials of your VNC connection to "Do not use any credentials" or use the "Connect with Options" menu and disable "Automatic Logon".
When the connection is established the credential picker dialog will pop up and offer the option to "Request Permissions" (it's in the lower left corner). Now when you enable this option, the remote user will be asked if he wants to allow the connection attempt and if the answer is yes, you should see the remote screen.
Tags: OSXFAQ VNC Apple Screen Sharing Apple Remote Desktop