Hey!
As far as I know, Unity requires graphic acceleration as of 10.10, which would explain why 10.04 works and 10.10 does not; you might have to install the ATI drivers using the restricted driver manager (which require a reboot, which means you cannot install them on a live cd / test run)
What I would suggest is installing side by side (so you can still boot into Vista), and trying the Gnome Desktop instead of Unity. You can do that from the login screen, at the bottom you have options for language, desktop environment and keyboard layout.
From there, you should be able to install the drivers under System / Admin / Additional Drivers, and be able to use Unity after you reboot.
Personally, I much prefer to use the Gnome Desktop even on a netbook, but I encourage you to give both a try
Good luck!