You shouldn't need a 'virtual' audio cable.
Very little cap+ here but setup here when trying DSD+ out a few months ago:
Audio from the radio to DSD+ (CC) via windows sound device line in
DSD (CC) outputs via TCP to FMP24 (VC)
FMP24 (VC) outputs to DSD+ (CC) via TCP
DSD+ (VC) outputs to speakers via normal windows sound device
DSDPlus.frequencies with entries for slots 1,3 and 5
Worked perfectly.
I then tried it with two nesdr 'dongles' to fee up the line in as it's usually used for something else. Using two 'dongles' was just as easy.
So then what output setting should DSD(CC) have? There is no parameter that I find, such as -o20001. I have now set DSD(CC).bat to -i1 (Line in) and no output. I run it and it decodes the control channel.
The FMP24-VC.bat has no parameter for input (such as -i20001). I have output as -o20002. When I run FMP24-VC.bat is says "Using TCP port #20002" below the config entries and at the bottom it says "Loading frequency data..." "Listening on TCP port 20002" "Trunk voice following active" "Idle".
The FMP24 tuning window is blank.
On to DSD(VC).bat,,,, -i20002 -o1 (speakers). Run it. I get:
Assuming FMPA/FMP24 is signal source; auto polarity detect is disabled
Initiating TCP connection to 127.0.0.1:20002...
Connected to 127.0.0.1:20002
audio output device #1 = 'Speakers (Realtek High Definiti'
audio output device #2 = 'VB CABLE (VB-Audio Virtual Cabl'
audio output device #1 (Speakers (Realtek High Definiti) initialized
P25 Phase 1 decoding enabled
Mono audio decoding initiated; press Esc to exit...
So there's my setup. The only part that works at this point is the DSD(CC) decoding the line in audio from the tapped scanner.