The "vault" in this poem is not a physical structure but a metaphor. It refers to the "vault of heaven," meaning the sky, or the "vault of time," meaning the passage of time.
Here's how it works in the context of the poem:
* "Two vast and trunkless legs of stone Stand in the desert." This sets the scene of a crumbling statue in the vast emptiness.
* "Near them, on the sand, Half sunk a shattered visage lies, whose frown, And wrinkled lip, and sneer of cold command, Tell that its sculptor well those passions read Which yet survive, stamped on these lifeless things, The hand that mocked them, and the heart that fed; And on the pedestal these words appear: ‘My name is Ozymandias, king of kings: Look on my works, ye Mighty, and despair!’” This continues to describe the ruined statue and the inscription on its pedestal.
* "Nothing beside remains. Round the decay Of that colossal wreck, boundless and bare The lone and level sands stretch far away." Here, the "vault" comes in. The ruined statue is surrounded by the vast expanse of the desert, which symbolizes the "vault of time" that has swallowed the once powerful king and his empire.
In essence, the "vault" in "Ozymandias" is not a tangible thing but a symbolic representation of the vastness of time and the inevitable decay that consumes even the most powerful empires and individuals.