Morningstar reviews Vanguard each year. From the recent report (Sep 2016) we understand that their robo offering isnt profitable YET!
"Personal Advsiory Services (PAS) had $36 billion in assets as of March 2016. Vanguard's scale allows the firm to offer its relatively high-touch service at such an attractive price. But Vanguard has implied that the service is not yet covering its operating expenses.
This points at a tension that exists for Vanguard even though it is mutually owned. Specifically, how should its scale best be used for shareholders' benefit? As PAS doesn't yet cover its costs, it is implicitly subsidized by fundholders. "