If you select lines and points to make a surface, then the surface is parametrically defined and when you export it to VCL the entire definition goes, so you have the lines and points and the full definition. The surface is the objects and the settings for the surface and the current triangle relationship. Changing the objects changes the surface. This allows you to edit the definition of the surface when imported to another file.
You can use the offset surface command to create a new surface or export to ttm and import to make an "internal" surface that is no longer dependent on the objects. When you export this surface to VCL you will have just one object, but you will no longer be able to edit it.
If you are using the takeoff workflow then the surface is defined based on the layers used to make the surface. If you export this surface it will carry the triangles but not the objects from the layers specified as "Design layers". The VCL will have the surface, but if you make any change that requires the surface to be rebuilt then the data needed is not present and the triangles are removed.
The same goes for a surface defined from a point cloud. The triangles go but not the point cloud. Any change and the surface is lost.
Formats such as landxml and dwg are not parametric and require that we create objects understood by other systems. You typically get more data but without the intelligence and object relationships. Importing data from other systems typically creates more objects than originally needed within TBC.