vino. Its only job is to share the currently open session which means you can't do what you want.
There are a couple of alternatives to VNC but I'll start with another VNC solution first. TightVNC is another open source server that, with a bit of kicking, lets you pipe a whole new GDM screen through VNC, effectively allowing you to login as who you want.
The downside to this is you don't get to remote-control apps running on the real computer screen as when you login to this, you'll get a brand new desktop session. You might just want to control whoever's logged in so this wouldn't be for you.
Unfortunately the only tutorial I've* been able to find is for Gentoo (another linux distribution).
*And when I say "I've", I mean
#ubuntu. I couldn't find any tutorials.
If you do just want to control whoever's logged in but hate having to configure VNC, I use
x11vnc over ssh from my phone (it's a linux phone). Here's the one command I fire off:
ssh email@example.com -L 5900:localhost:5900 "x11vnc" | vncviewer
This tunnels the VNC server's connection back over the SSH to a local port so you connect from your client to localhost:5900. The
| vncviewer bit just opens the VNC client at the same time. That might be specific to my phone, so you might need to change that to whatever VNC viewer you use.
There are other options perhaps better suited to the task. NoMachine/NX/FreeNX/etc are RDP-style servers that allow logins but they need a NX client. The benefit over VNC is it's really fast and quite low-bandwidth.
The Ubuntu help page should get you started but I'll warn you, it's not a short journey.