Yes, the advice to use only one virtual processor core is incorrect. I'm saying that the conditions which cause the advice to appear should not be happening. We'll need to dig into it to determine why that message appears.
Recent versions of OS X definitely require at least two cores to operate reliably. That's the warning you see on the Processors & Memory pane when you configure the VM for only one core.
Under certain conditions, some very old versions of OS X might run unreliably when they are running on more than one core. The incorrect advice appears because we somehow think you are running an older version of OS X, and we attempt to fix the problem which could cause the old version of OS X to run unreliably. Usually we silently fix the problem, but here we are incorrectly reporting that we fail to fix it, because it is not actually an old version of OS X at all.
Ignore the "Your Mac OS guest might run unreliably with more than one virtual core" advice... it should not be appearing.
Thanks for the report. I'll get back to you if I need more information.
Cheers,
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Darius