In the latest narrative preview, Saint-14 is about to go on his Crusade against the fallen houses and asks Osiris to lead as the vanguard commander. This is his reply. [quote] “You want to give me control over the databases? The Vaults? Jurisdiction over Owl Sector, access to the Last City grimoire?”[/quote] I guess this is how ghost knew about the books of sorrow. But that begs the question: How much about the lore do the Vanguard know?
The lore they knew was probably at the discretion of the Speaker. Now that he's gone, the Vanguard probably have full access to the grimoire.