Soviet psychologist Lev Vygotsky made the connection more explicit; he found that children begin by speaking their stream of consciousness aloud to inform other people, and eventually learn to suppress that stream into nonvocal (subvocal?) thought.

That doesn't sound right. I've heard mentions of children who only start speaking at a late age, but when they do, they skip right to speaking in fully-formed sentences. I'm not sure whether I understand correctly what you're saying here.