The "only the president can launch a nuclear strike" myth. If it was that easy you'd only need to blow up the White House with one nuke and take out America.
There are "continuity of operations" procedures to prevent a "decapitation" strike killing the President & VP, thus preventing the US from retaliating [0]. This was a plot point in Tom Clancy's novel Debt of Honor [1] (which I distinctly remember reading right before 911, as I had to commute via public transit and commutes were so long I read books).
I'd assume the VP has their own nuclear suitcase. Unless they're forced to choose a random number, this probably makes impersonation easier as it's n points of failure where n is the number of people who have their own nuclear suitcase.
