Can a Deputy President of Kenya (Gachagua or any other) act as a Cabinet Secretary? NO.
He or she may be assigned tasks that are undertaken by a CS or day to day basis but can he/sign official papers as Acting CS? No.
Article 152 of our dear constitution establishes a Cabinet. It clearly separates a Deputy President (152(1)(b)) from CSs, who must not be fewer than 14 and not more than 22 (152(1)(d). Even when there are 21 CSs, the DP has no constitutional mandate to step in and act as the 22nd CS.
Musalia Mudavadi is just a CS like all the other fired ones, that’s why he can hold all the 22 dockets (such a daunting task for the guy as we know him… Forget about the Prime whatever. He was simply vetted as a CS. The added unconstitutional title is just to make him feel good, the same way we have class prefects who are just students.