If they had pneumonia, and therefore fluid on the lungs, the voice would sound high-pitched, and the vibrations would be reduced. Maybe not infallible, but pretty slick, right
Oh dear, we'll leave aside that vocal resonance is a better test, and that you wouldn't test for vocal fremitus if the rest of examination was normal anyway, no, the point here is that the vibrations would be increased in pneumonia!* I'd expect my medical students to know this, let alone a GP. What a clown.
* Don't give me any obfuscatory crap about a parapneumonic effusion - if that's what he meant that's what he'd have said.