Microsoft XP License Prohibits VNC
That's nonsense. VNC does not allow you to "setup a Windows box and have 50 Linux desktops connect to it"--VNC doesn't magically transform a single user Windows machine into a multiuser machine. I wish it did, but XP is such a primitive system that that's not easily possible. As far as Windows is concerned, VNC gives you the ability to remotely control a machine into which you are logged in, no more.
In any case, what matters is not whether VNC lets you do this but that Microsoft tries to impose such restrictions. It's just another indication of how much they are trying to milk and control their customers. Any rational buyer should run from that kind of company, and this should be added to the long list of anticompetitive practices to be investigated.