I would like to know which software of what is the procedure to create a VNC server such that each user can have access using its own user/password combination ?
You can use X11VNC for that. I don't know if there's any particular reason why you have to use VNC, but if you just want a remote desktop solution over SSH, you may want to have a look at http://x2go.com, which is an NX-based solution to provide just that. It has some nice features, such as PulseAudio forwarding. It's very fast too – even over slow connections.