Instead of putting the ScatterView in a Viewbox, you should do the following:
After a ScatterViewItem is placed on the map, transform its Center point (which is in viewport coordinates) to a location in world coordinates (latitude and longitude). The ArcGIS API should provide such a transformation.
When the map is zoomed or otherwise transformed, transform each world location back to viewport coordinates and set each ScatterViewItem's Center property accordingly.
In order to store the world coordinates for each ScatterViewItem, you could create an attached property.