Perhaps Westmoreland could come up with an effective plan to invade north vietnam, let's say in '67. He sends it to Washington, but it is not turned, most likely after Tet. Out goes Johnson, in comes Nixon. Westmoreland submits his second draft around '70-'71, and this is turned down, maybe on the influence of Kissinger or Nixon. Westmoreland protests and he is removed from command, and threatened if he reveals anything. Watergate comes along, and Westmoreland (either still in the military of governor of South Carolina) subtly leaks the situation. Nixon resigns, Ford is in, holds off on nominating a veep until he pardons Nixon, loses popularity, then nominates Westmoreland to win some support back. Either of the assassins in California get Ford, and Westmoreland is the next President.
Of course I don't know too much about Westmoreland or any plans to launch a ground invasion of Vietnam, but it does fit the "one hand tied" mentality.