The login gui would be part of the client, not the server.
If you haven't gotten very far yet with VNC, may I make a suggestion? NoMachine's
nxserver and nxclient provide the remote desktop capability, but with less configuration and better performance. There are clients for windows, mac and linux. The clients have a connection wizard which works pretty well, and a login gui. Free, but not open source. There is also an open source clone called freenx, which I haven't tried lately - not sure how stable it is yet.