You can't say 1:1 directly that cheap credit = rise of Vanguard, but index funds have gotten really popular since 2008. The near-zero interest rate was the environment, and that environment was built on the Fed's policy goal of the Wealth Effect (belief of growth of assets boosts demand & spending). So, given that you could pretty much throw anything at the dart board among the giants and it would go up, indexes performed exceptionally well and capital poured into them. They were also initially seen as 'safer' post 2008 meltdown, but I think as the years passed it became less about this
But their growing concentration (esp. regarding Vanguard) has also worked to naturalize some of the largest companies as a 'group.' This kind of frame became sort of unavoidable amid all of them rising, so it's hard to separate the 'cheap money' environment from Vanguard's influence growing - especially since this macro environment also allowed for their entry into other ballooning assets like real estate w/ their ETF (which has been controversial)
The problem with this growing concentration is that, as Vanguard becomes a major holder of every top stock, then all those grouped assets are going to start moving more and more in lockstep the further the concentration grows. And that's less and less of a market to me, and more just a artifice unrelated to actual profitability. In all, this is another child of the post-Great Recession cheap money environment, although how directly you want to tie them together is debatable