I'll paste part of my support thread answer here so that customers with similar questions may read it:
"As far as I understand you have a custom diagram shape (which inherits our RadDiagramShape) and you're using the VertexPoints collection to create its geometry. This all works fine initially but after resize you'd like to update the VertexPoints collection with the new absolute point values. Can you please confirm if this is indeed your scenario?
If it is, then you need to have in mind that WPF doesn't change a geometry after a resize. Instead it scales it accordingly based on the Stretch property of its Shape presenter, e.g. Path. By default the RadDiagramShapes are defined with Stretch=Fill and therefore the points in their Geometries are not absolute but relative to the current size and you'll have to manually calculate the new points after resize. To do this you'll need the initial relative positions and the new width:
I've attached a sample project demonstrating approach this so could you please examine it and tell us if this is what you need?"
Point(relativePoint.X * newWidth, relativePoint.Y * newHeight));
We're looking forward to hearing from you again.