Hello,

I have been looking into your question and created a small sample that demonstrates a command binding to XamNumericRangeSlider's ThumbValueChanged Event.
I have added a XamNumericRangeSlide with two thumbs. The thumb values are bound to members of the View Model. There is also a text block, which shows the difference between the thumb values.
In the View Model I defined an ICommand - CalculateRangeSpanCommand, which is initialized as RelayCommand:

this.CalculateRangeSpanCommand = new RelayCommand(param => CalculateRangeSpanCommandMethod(param), param => true);

The sample command method's purpose is to refresh the text block, displaying the difference, upon changing a thumb value. It is defined as follows:

public void CalculateRangeSpanCommandMethod(object parameter)
{
    if(parameter != null)
    {
        ObservableCollection<XamSliderThumb<Double>> Thumbs = parameter as ObservableCollection<XamSliderThumb<Double>>;
        RangeSpan = Math.Abs(Thumbs[1].Value - Thumbs[0].Value);
    }
}

The Command is bound to the XamNumericRangeSlider's ThumbValueChanged using the Interaction class. I am also passing a parameter to the command, to be able to retrieve the Thumbs collection in the method:

<ig:XamNumericRangeSlider Name="xnrs1">
    <i:Interaction.Triggers>
        <i:EventTrigger EventName="ThumbValueChanged">
            <i:InvokeCommandAction Command="{Binding Path=CalculateRangeSpanCommand}"
                                                                  CommandParameter="{Binding Path=Thumbs, RelativeSource={RelativeSource AncestorType={x:Type ig:XamNumericRangeSlider}}}">
            </i:InvokeCommandAction>
        </i:EventTrigger>
    </i:Interaction.Triggers>
    ...
</ig:XamNumericRangeSlider>

I have added a Relay Command class implementation to the sample, however, you could modify it according to your requirements.

Attached could be found my sample for your reference. Please test it on your side and let me know if you require any further assistance on the matter.

Sincerely,
Bozhidara Pachilova
Associate Software Developer

5875.XNRSCommandBinding.zip