If the program offers no audio source control, then it likely uses the default audio input device, so just set the default to VB Cable.
Or use Stereo Mix.
I have found that only works sometimes on older programs and did not check for Win MDCD but I found there is another solution that I had forgotten SDRTrunk has an MDCD 1200 decoder. That program will allow one to select the input (to VBCable once installed) and is probably the best way to go as it does not involve changing the default audio device (the program can be used with VBCable without an SDR). That programs also allows him to do other things.
I use SDRTrunk to decode Lojack base stations (here a stolen device is rare) on 173.075. The writer of the program put that in when he discover he easily could, Not all that interesting but once identifies where the individual station are, it might be used to check on propagation. In that case, I feed it audio via VBCable though the program is not controlling an SDR.
For those interested, here are some of the Lojack base station locations and identifiers:
011F Bull Run (Haymarket), VA
014E Bethesda, MD
01AE Baltimore, MD
021F Midlothian, VA
022F Philadelphia, PA
02AE Prince Frederick, MD
031F Emporia, VA
03DE Rosenhayn, NJ
802F Wyndmoor, PA
805E Rising Sun, DE
80AE WPLP844 Clarksville, MD
811F WQTM566 Fredericksburg, VA
812F WQKE793 Reading, PA
814E WPED308 Washington, DC
81AE WPLP844 Frederick, MD
82AE WPLP844 Hillsville, MD
831F WQTM566 Suffolk, VA
83DE WNRU940 Atlantic City, NJ
Note that the last two letter of the identifier apparently identify the state (or adjacent location in that state's network).