I emailed the developer for VAC to ask this same question. He responded:
You can add "-s -k 30570681-0a8b-46e5-8cb2-d835f43af0c5" to the
setup.exe command line.
I have confirmed that this worked using VAC 4.64:
.\setup64.exe -s -k 30570681-0a8b-46e5-8cb2-d835f43af0c5
The installer returns immediately and the process installs in the background. However the installation only took about 10 seconds at most. If you needed to catch a failed install I would suggest monitoring the active audio device and timing out based on that.
Installation log file (on my system) went to C:\Program Files\Virtual Audio Cable\install.log