This is known issue affecting vCenter Server 6.5.
Currently, there is no resolution.
To work around this issue, deploy a temporary vCenter Server 6.0 to import and export the vDS:
- Create a vCenter Server 6.0 for temporary use.
- Export existing vDS from vCenter 5.5.
- Import the vDS to the new temporary vCenter Server 6.0.
- Export the vDS from vCenter Server 6.0.
- Import the vDS exported from vCenter Server 6.0 to vCenter Server 6.5.