What VNC service would you recommend

You can't enable XDMCP in XFCE4. XDMCP is something that is enabled in the display manager, not in a desktop environment. As you didn't tell us which display manager you use (GDM? XDM?), I can't help.

Note that some display managers (e.g. SLIM) don't support XDMCP at all.

No such tool exists.

And before you run off and try to write a script yourself to do something monumentally silly (like trying a bunch of different credentials on 500 VNC servers and reporting the results back), the only solution to this that isn't going to be ridiculously painful or involve a bunch of manual work is below.

1. Write a script/GPO to uninstall all the VNC servers in your environment.
   • My experience with those VNC servers tells me you can check against registry keys to get the specific application and version installed, and then use that to call the (proper) uninstaller.
2. Determine a standard VNC server deployment you desire.
   • Including the credentials you wish these servers to have, of course.
   • Depending on which VNC server you decide to push out, it may be easier to just install the server with the installer, and then copy .ini files containing the settings and credentials to all your clients (via GPP).
3. Write a GPO to install the VNC server to all your clients.
   • Probably best to do with a computer startup script.
   • Again, use a registry key to check if it's installed before calling the installer.
4. Update script as needed for new versions.

This is going to be vastly better than trying to deal with multiple VNC server vendors in multiple configurations, and should even be relatively easy to manage, compared to what you have now, which I wouldn't even try to deal with. You're definitely in a situation where it's going to be vastly easier to build right from scratch than repair.