Not in any way as easy as iChat's "Screen Sharing" option, but it is possible. VNC would probably be the best option both ways, but it takes some doing to configure (especially if you're both behind NAT routers).
EDIT: VNC, not VLC. Brain fart.
We're both in the same network.
Do I just install a VNC client on the Vista PC and then use the 'share screen' button in the Finder to start the screen sharing app?
Install the VNC Client, go to Finder > Go > Connect To Server
and type:
vnc://VistaIPAddress
It should be noted that the OP has to install the TightVNC service, not just the client, to connect to Vista.
The client is only a viewer. The service is the server.