how to explain

"That way, even if Vanguard goes bankrupt, this will not affect the underlying conservative investments."

Vanguard non-retirement accounts as I understand, are not FDIC insured. how much does vanguard insure your investment? that's the most you can get if vanguard goes under.  i don't know how those brokerage firms works, they must use client's investment as leverage to invest just as anyone else would. so there is a chance they will fail. hence, your underlying investment will not be kept intact if vanguard fails.

also can you buy gold to stay safe? interesting enough, not really. I heard anyone buy gold bolliuon (how to spell it?) or maybe even gold coins have to register. US government in 1930s and 1970s confistigated gold bought/stored by individual. however it's important to point out that was back during the gold standard time. so this may not happen again.

call me a pessimist, i see no way around but to stick with the bank system, and hope things will work out.
at the same time, i think it is only safe to hold US treasury, not for any potential gain, but capital preservation.