Recently some manufacturers of notebook computers, particularly those produced or based in China, have been shipping laptops with the stereo mix feature disabled in firmware. A lot of discussion can be found online about it. On my particular ThinkPad, it can be enabled somewhat by a registry hack, but you have to disable the onboard microphone in order to use stereo mix, so it's not a great fix.
I think the manufacturers really had no idea how much they'd aggravate a whole swath of their users with this policy. I've read complaints ranging from disappointment over no longer being able to do karaoke with a newly purchased laptop, annoyance at not being able to do advertising voice-overs, to some real ire at not being able to perform audio mixing and recording by a church music director.
Virtual Audio Cable (VAC) has a reputation for being a tough program to learn and use. It can be. If you like to dig deep into the plumbing, there's enough theory, principles, background, associated content, and practices in the Help Guide to satisfy and bemuse. Here we're taking a more direct approach. We'll talk about what we need to set up VAC on a laptop that lacks the stereo mix feature and how to use it to record one or more sound sources, including "what u hear" and microphone mix, on a notebook.