I currently run Sierra on a late 2013 MBP (Retina). I want to set up an environment where I can learn and experiment with programming in a Mac environment without the risk of doing damage or making a mess, so using a VM sounds like the way forward. I don't know much about VMs beyond setting them up; what I'm wondering is - applications and tasks aside - would running a VM of Sierra on top of the host installation effectively double the amount of system resources being used or do VMs not work that way?
Not at all a downside. The VM does add a slight performance penalty, but at idle, you don't do double work. Also, you can always quit apps on the host or the guest os to free up resources. For isolating changes and tests, VM are great. Especially igpf they support snapshots and cloning setups to avoid tedious configuration steps.