Can I open or import layout templates designed with ArcGIS Pro 3.x in VSP Designer, if the VSP Engine Tools are published on ArcGIS Enterprise 10.x?

Yes, but it is NOT recommended to process newer CIM (Cartographic Information Model) v.3 layouts in ArcGIS Enterprise 10.x environments, because the underlying CIM is v.2. Note that since the layout template will be converted to a PAGX CIM v.2 layout as soon as the layout template is saved or exported in VSP Designer that CIM v.3 specific layout template configurations will be lost. Furthermore, layout templates might become corrupt if mandatory CIM v.2 configurations are missing.