@frankk, thanks for the references. I eventually got a chance to have a look through these and the related references (here and here) you collected. For the most part, I’m not sure they really clarify what vacīsankhāra is (those suttas emphasize that it is what underlies or what impels speech, often the translations opt for the choices/intention nuance of sankhāra). None of that surprises me (right intention after all is just before and leading onto right speech in the eight-fold path). These references I think imply vacīsankhāra is a precursor to speech, but am not sure it implies much more than that.
However, I did find your AN 3.60 translation relating to telepathy very interesting. It describes telepathy at the V&V level and then also at a deeper level of the mind. I must admit that, as you described there, the use of words like “sotam = ear, sadda = sound, sutva = hear” in relationship to the V&V-related telepathy described is definitely a point in favour of your V&V interpretation as (if I’m understanding you correctly) literally including mental vocalizations.