Тhe same approach as the one explained in the Zarko's reply can be used also in an MVVM scenario. Basically, you can create a custom container shape that derives from the RadDiagramContainerShape class and override its CalculateContentBounds() method.
FixedContainer : RadDiagramContainerShape
System.Windows.Rect CalculateContentBounds(System.Windows.Rect newShapeBounds)
However, in order to use this container in an MVVM scenario you will also need to create a custom diagram and override its GetShapeContainerForItemOverride() method. You can find an example that demonstrates such scenario in the Create Custom Shape
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