Arches may be synonymous with vaults, but a vault may be distinguished as a continuous arch forming a roof. Arches appeared as early as the 2nd millennium BC in Mesopotamian brick architecture, and their systematic use started with the Ancient Romans who were the first to apply the technique to a wide range of structures.
From what I have been able to gather through this thread is that the first use of the arch in Egypt, albeit a vault of sorts, is evident in the Pyramid of Menkaure. Being such a hit, it was then used in the Mastaba of Shepsekaf. Timelines are often a little fuzzy in the Dynastic periods, but no mention is made in Wiki about high quality stone arches in Egypt which have probably outlasted their brick Mesopotamian counterparts.
And the Egyptian versions were just about perfect from the get-go.
I'm getting a feeling that something could be missing here.
Palace facade coffers, Mesopotamian arches, abandonment of the Pyramid architecture, Djedefre wandering off to Abu Rawash, no statuary of Khufu.
Maybe I'm reading too much into it.