One thing I recently discovered that gave me some pause for thought is what happens if you buy a vanguard ETF and either the provider or vanguard itself somehow gets into trouble. For example, what happens if the ETF provider is actually loaning out the underlying stocks and gets caught by market movements. Apparently in the UK at least there are quite a few scenarios in which the FSCS 85k guarantee would not apply (in contrast to a cash savings account). See e.g. discussion here: https://monevator.com/physical-etf-risks/
I don't think this kind of situation could occur in the US because rehypothecation of shares underlying ETFs isn't allowed. See also https://www.bogleheads.org/wiki/Vanguard_safety And worst case if something did happen it would be bailed out.