Hey that's a neat little box!! MIDI Solutions Foot Switch
Did some Googling and I see how it gets powered from a MIDI out port. Below is a link to a schematic that shows basic MIDI hardware wiring.
In Figure 1 there is shown a MIDI OUT connector. To pin 4 of this connector there is +5 vdc internally supplied from the host device through a 220 ohm resistor relative to the ground return on pin 2. It is this +5vdc that can power an external device, but may depend on the current draw of the device. I would assume the current draw to operate the MIDI Solutions Foot Switch is within the MIDI specification and should normally be OK, but no guarantee with that.
In your case presently the +5 vdc is supplied to MIDI Solutions Foot Switch from the MIDI OUT of pin 4(+5vdc) and pin 2(ground) of your Voice Works unit. On your VL Rack this 5vdc needs to exist to be able to provide a current loop source to operate an Opto-Isolator LED at the MIDI IN of a MIDI data receiving device. As Craig had suggested make sure the USB MIDI is disabled and the MIDI OUT port is enabled. This should ensure that the +5vdc is present.
I did take a few moments to measure a couple of pieces of MIDI gear I have. They all had a voltage present at the MIDI OUT pin 4 connection. However one device only had about 3.3vdc and according to MIDI Solutions this may not be enough to operate the footswitch. The MIDI Solutions Foot Switch is shown to need 3.5vdc to operate properly. Perhaps the VL Rack voltage does not meet this criteria.
Roland DR-880 3.3vdc (not sure why this is what it is, but does work connected MIDI devices OK. 3.3vdc is a common voltage for CMOS logic devices(?))
Roland XP-60 5.0vdc
Alesis SR-16 5.0vdc
If Craig is still listening, perhaps he could have his engineers check....
- the MIDI OUT connector voltage at pin 4 relative to ground is at or around +5vdc
- check that the MIDI OUT pin 2 is internally connected to ground. There should only be one ground connection between MIDI devices to prevent ground loops. Usually the MIDI OUT pin 2 is grounded and MIDI IN pin 2 is not connected inside a device.
I do have a MIDISport UNO, but prefer using the MIDI IN's/OUT's on a M-Audio Interface card in the computer.