If there is an opening in the VP spot, the President nominates a new VP. But that VP still has to be confirmed by both houses of Congress before he can be sworn in.
The only way Johnson becomes president is if a) Trump dies, Vance nominates him, and Congress confirms it, or b) both Trump and Vance kick the bucket at the same time which automatically puts Johnson in the White House.
If there is an opening in the VP spot, the President nominates a new VP. But that VP still has to be confirmed by both houses of Congress before he can be sworn in.
The only way Johnson becomes president is if a) Trump dies, Vance nominates him, and Congress confirms it, or b) both Trump and Vance kick the bucket at the same time which automatically puts Johnson in the White House.