Can I open or import layout templates designed with ArcGIS Pro 2.x in VSP Designer, if the VSP Engine Tools are published on ArcGIS Enterprise 11.x?

Yes, but the layout template will be converted to a PAGX CIM v.3 layout as soon as the layout template is saved or exported in VSP Designer. This means that you will no longer be able to open the layout in ArcGIS Pro 2.x