Unlike its peers, Vanguard is owned by its funds — and ultimately its investors — so as money rushes in, expenses are persistently reduced, resulting in perpetual savings for the legions of Vanguard clients.

This idea of working for investors, as opposed to shareholders, has become dogma, so much so that the firm has been criticized in some quarters for not investing enough to keep up with the trillions of new dollars rushing in.

Mr. Buckley, like his predecessor, is a culture carrier and has a lean intensity that can be seen in many of the firm’s top leaders, not least Mr. McNabb.

“This place gets in your blood,” said Mr. Buckley, who has been Vanguard’s chief investment officer since 2013. “The opportunity to serve our clients well — that is what keeps us here. It is an awesome responsibility.”

Mr. Buckley joined Vanguard in 1991 as an assistant to Mr. Bogle, and he said that he has never considered leaving the firm.

Still, as all of Wall Street is struggling to grapple with Vanguard’s growth and its dominance in passive investing, Mr. McNabb’s decision could be seen as a pre-emptive strike of sorts to lock in a new generation of leadership at the firm.

Also promoted on Thursday was Greg Davis, who oversaw the firm’s fixed income division. Mr. Davis, 46, will succeed Mr. Buckley as chief investment officer when he takes formal control early next year.