If Coinbase goes out of business, you'll lose all your money. Coinbase is a startup, and >90% of startups fail. "Vault" is a marketing term designed to make people feel comfortable for no reason.
Given this major disadvantage, what benefit does Coinbase Vault offer to outweigh it?
Coinbase isn't running a fractional reserve. Barring fraud, everyone would get their BTC back even if it fails.