There are several potential problems I've heard of getting VNC to run on Vista. There's firewalling which may prevent connections. But the more interesting one is the "black screen" problem, which, thankfully, is easy enough to fix. I'm running TightVNC, but it probably is the same for RealVNC and UltraVNC. The fix is this -- turn OFF the VNC service from the control panel/services menu. Services in Vista DO NOT have access to the console by default (there's probably some way to get it to work, but I'm not familiar with it). Once the service is off, launch the VNC server directly from the desktop. Now people can connect and they don't just get a black screen.
Also, if your fonts are coming through really, really fuzzy, that means you need to turn off JPEG compression. My guess is that previous VNC versions were able to detect direct font writing and send them as discreet font updates, but that Vista's rendering enginer requires that they get transmitted as images. Anyway, just turn off JPEG compression and it will work fine.