The "either" establishes the precedence:

(VP and (either Cabinet or Congress))

But, it only requires a majority if the President willingly goes along with it, or is unable to respond. If the President makes a counter-declaration that he is fit to serve, both the House and Senate must vote within 21 days, and a 2/3rd supermajority vote is required in each.

The "either" establishes the precedence:

(VP and (either Cabinet or Congress))

But, it only requires a majority if the President willingly goes alone with it. If the President makes a counter-declaration that he is fit to serve, both the House and Senate must vote within 21 days, and a 2/3rd supermajority vote is required in each.