Call normals update and re-tesselate the BMesh before generating derived mesh for vertex parent. this is needed for proper display of mesh in edit mode. Tried to use EDBM_update_generic, but it gave artifacts due to it doesn't update normals. usually it's not a problem, because it's used at the end of operator and all needed data is handles by depsgraph. It doesn't work for vertex parent because derived mesh is being created here outside of generic object update, so one extra manual step seems to be needed here.