Ds2 recently covered propositional phrases, and had a homework for it, but he is half way through year 6. Doubt "by virtue of" would arise, but maybe just show your dc, that they do not have to be a location but in nearly all cases have a simple preposition word, e.g...the word "by" in your example, as part of the phrase. So teach him to recognise one rather than just use one without realising he has used it.
There is a small chance that a comprehension may ask the pupil to find and copy a propositional phrase, I have seen this sort of thing before, but do not ask we where, it just rings a bell in my addled mind.
Thanks Southbucks3: that's really helpful.
Hope you don't mind that I enjoyed your typo. It's been a long time since I've had to find and copy a 'propositional' phrase!