Hey Tom,
You can definitely add/remove vCPUs from your 2008 R2 VM. It sounds like you want to script it and do that hot though, and while that's possible to configure and do unless you're running Datacenter edition on that VM hot add CPU is not supported by the OS. You also can't hot remove it regardless, no OS currently supports hot removing RAM or CPU unless something has changed.
For your needs the best way forward would be to shutdown your VM, right-click select Edit Settings, and under CPU just switch cores from 1 to 2. You can just reverse this when you don't want the extra vCPU anymore. You shouldn't need to remove that single vCPU from your VM unless you're over-saturating your physical host's cores.
I'd suggest setting and keeping that VM at 2 vCPUs if it needs it.
~Brandit