Generally you are correct. Stocks bought via a broker are supposed to be ring fenced assets and as such the FSCS only helps you in one of the following situations:
- You happen to have a major amount of money in your current account, while your broker goes bust.
- Your broker somehow manages to lose your stock, which is not totally unheard of (especially while buying, selling or transferring to a different broker) and refuses to compensate you.
- The broker commits fraud and basically steals your ring fenced assets (as they are legally your property).
Ring fencing is also not limited to your broker, as most funds offered for sale in the UK are UCITS funds. The UCITS standard
guarantees ring fencing of all assets inside the fund and also supervision of the ring fencing by an independent custodian. 
Overall stock brokers and fund providers depend on the trust, the general public has in them. They can't afford to loose that trust, so they have a great incentive to be transparent about their asset management practices and often offer extensive documentation. 
So will the FSCS save your butt if John Bogle turns all evil?
The problem with that, is that Vanguard has $5.2 trillion in assets under management, as of January 2019 , while the proposed budget in 2019/20 for the FSCS is £79.6 million.
More seriously put: If really bad stuff happens to the economy at large, you can't take for granted that the FSCS will refund that £85,000 to you.
Does it make sense to split your portfolio across multiple brokers?
There is no easy answer to this question, as it is a personal decision, but I would say most people opt to keep their stocks with a single broker (and often also with a single ETF provider) and trust in proper ring fencing being carried out.
For other deposit-like assets in their portfolio (like CDs) more people opt to use multiple banks and only invest funds limited to the £85,000 (or €100,000 in the €-room states), as far as my personal experience goes.