Okay. That's great.
What you can do is use the keyboard to set your desired device as default making note of all your key presses. When the applet initially appears, you may press the down arrow key to select the first device in the list. Press down arrow again to select the item in the list. Once you have the correct item selected, press tab to navigate to the Set Default button. I had to press it twice.
Once the button is selected, the enter key make the device default.
Next, press tab until the OK button is selected. I had to press it two times.
Finally, press enter to close the applet.
Here's example script to do this. You will want to change it according to your noted key presses:
Set oWSH = CreateObject("WScript.Shell")
oWSH.Run "rundll32.exe shell32.dll,Control_RunDLL mmsys.cpl", 1, False
WScript.Sleep 2000
oWSH.SendKeys "{DOWN}{TAB}{TAB}{ENTER}{TAB}{TAB}{ENTER}"
Save this as a file with a VBS extension. You can test it out by executing it to be sure it works. Note that the last line has a single down arrow key press. You will want to change this to the number of times you had to press the down arrow key to get to the device you want to be default.