To be honest Binding a control's property in XAML to a CLR property in the ViewModel is very common WPF question and it doesn't really relate to our RadProgressBar. What you really need is something like:
In your ViewModel you can do:
_currentProgress = value;
If demanded you might also create a Minimum and Maximum properties in your ViewModel and bind to them as well. Let me know how this works on your side and whether you need some additional help.
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and share your thoughts.