TG--
You raise legitimate questions regarding Newman's take, or premise, that there was a high-level CIA plot to remove JFK from office, somehow connected to Cuba, Vietnam and NSAM 263.
We keep reading speculations about such a plot...but nothing solid.
Keep in mind, JFK was an ardent anti-communist, and Vietnam in 1963 was just not that important. And JFK's plan was to Vietnamize the resistance to communist aggression, not walk away.
Cuban exiles angered at the Bay of Pigs? Maybe. Alpha 66?
LHO as G2 or KGB pawn? Not explored enough.