VNC displays a desktop, not a display manager like GDM, KDM or XDM. The only way to setup what you are talking about is using XDMCP, but it's a pain, unless you use LTSP!
My suggestion, for what it's worth, is use a FreeNX Server, search about for it. It works really well, and though it does not display a display manager, it does allow the client app to ask for login details. It's also inherently more secure than VNC, because it uses ssh to encrypt connections, and the response on the desktop is, IMHO, far superior to VNC.
Just my two cents!