The plot thickens!
I think is is safe to assume the transformer will be located on the (new) concrete pad.
Regarding the new vault, I have two (speculative) thoughts:
1. This new vault will replace an existing, smaller vault that is located under/near the sidewalk north of the site (see posting dated May 20). Reasoning: the existing vault is too small to allow the additional connections needed for the Supercharger.
2. This new vault is for future electrical expansion and has something to do with the trench lines that were marked on the asphalt next to the transformer pad. The trench lines were marked, but no trench was ever dug.