Several users simultaneously on a Mac mini
Not sure how it was 2 years ago when this question was asked but I confirmed that this work out of the box for me now.

Mid 2010 Mac Mini running OS X Mavericks 10.9
Client machine tested: Windows 7 Pro using TightVNC Viewer, iPad using iTeleport.

Both client devices were able to connect to Mac Mini via VNC and log in to different user accounts at the same time.

On every VNC session, it will show the mac login window which user can choose to access specific user accounts.

Though I would agree that it might be using up more resources when multiple users are connected at the same time, furthermore using for development.