Ah. I see someone else has spotted the same thing.
"The Vacancy Reform Act bars Trump’s installation of the unconfirmed Matt Whitaker to act as attorney general to control Robert Mueller unless Sessions truly “resigned” rather than being fired. A federal district court could be asked to see through the ruse and hold he was fired."
and "In addition, putting a mere staffer in the role of Attorney General — not to meet a temporary emergency but to oversee a special counsel’s ongoing probe of Russia’s attack on our presidential election — is inconsistent with the Appointments Clause of Art. II of the Constitution."
- Laurence Tribe