This feels like an article that will be shown for irony points in a documentary about Vanguard's collapse a few years from now.
Keep in mind also that "growing" in this context doesn't refer to huge profits (as far as I know). As far as I can see, as a non-expert, Vanguard just offers the same products as everyone else, except at half the yearly management cost. I mean, when you're looking for some index fund, and Vanguard is one third the cost, is it any surprise they're taking away customers from the incumbents?
EDIT: For example, here's TLT (0.15% p.a.) vs Vanguard's long term bond etf (0.07% p.a.): https://personal.vanguard.com/us/funds/snapshot?FundId=3144&... https://www.ishares.com/us/products/239454/ishares-20-year-t...