They own their own management company, and the management company charges lower fees than the industry norm because it's a captive. I'm a fan, since I have money in Vanguard index funds, but I know they are getting sued over it by some lawyers who think they are basically competing unfairly by owning the management company.
Btw, Vanguard has a really long history, and is a descendant of one of the original, if not the original mutual funds.