When using the V-Amp Design editor software for Windows, you will need to use both the MIDI send and MIDI return connections - at least on start-up of the software.
The software sends a request for a device ID - which must be returned to the PC via the second MIDI cable. So you need to use 2 MIDI cables.
To check the status of MIDI THRU" / "MIDI OUT" in your V-Amp model enter "EDIT" mode by pressing both of the arrow keys (bank up bank down) and selecting "MIDI" (key A).
In this configuration the "TAP" key can be used to toggle between MIDI THRU (LED off) and MIDI OUT (LED on).
To use your V-Amp model with the editor software you must select "MIDI OUT" (LED off).