Strictly speaking you cannot do this using VNC.
VNC just duplicates an already running session, so whoever logs on using VNC - everybody will see the same desktop. And there must be someone already logged in under which account VNC is running.
When you really need/want a new session then you need to install an X-Server on your client from where you are connecting (here I am assuming you are using Windows) and open a new session (e.g. using ssh).
There are two things I can recommend: Xming and nomachine.
If you do not need a new session with Desktop you might live with
nomachine can give you more.