what is the historical background of the sennacherib vault symbol in the royal arch and how do standard commentators interpret its meaning

The supplied library sources contain no reference to Sennacherib or a symbol associated with his name. They do, however, detail the historical background and symbolic interpretation of the Secret Vault in relation to the Temple at Jerusalem.

The tradition of the Secret Vault derives from Talmudic legend rather than documented history 1. According to these Rabbinical accounts, King Solomon constructed a subterranean chamber beneath the Temple to safeguard sacred secrets, built with seven arches supported by pairs of pillars 1. While the specific narrative is mythical, topographers such as Barclay and Prime confirm that extensive subterranean vaults do exist beneath the Temple Mount 1.

In Masonic symbolism, the secret vault represents the grave and the passage into eternal life 1. Ancient mystery rites conducted initiations in subterranean structures to symbolize death, holding that divine truth is uncovered only after descending beneath the earth 1. Mackey explains that Freemasons adopt this same imagery, treating the surface structure as the transitory temple of mortal life and the secret vault as the passage through death required to discover the sacred deposit of truth 1.