At the convention, delegates make the rules.
It is no secret that Cruz has been electing delegates in many states where Trump won the presidential preference but was too lordly to recruit mere party delegates. One estimate is that a third or more of Trump’s delegates really want someone else, giving the anti-Trumps a strong majority of delegates at the convention. Most are conservative or party activists who neither Trump nor the party establishment can control.
.. Does not Republican Rule 16(a)(2) say that if a delegate tries to vote for someone other than the candidate that won a binding primary that vote shall “not be recognized” by the secretary of the convention? Yes, but the delegates select the secretary.
Rules expert Republican National Committeeman Curly Haugland believes that Rule 37(b) trumps (sorry) 16(a) so that delegates can demand a roll call and vote any way they want. Who will decide? The chairman of the convention, who also is selected by the delegates, will make that decision.
.. the delegates could easily choose whomever they desired for vice president, no matter what the nominee wanted, and they should. Liberal Democrat Adlai Stevenson was forced to accept conservative Sen. John Sparkman as vice presidential nominee in 1952 and threw the VP nomination open in 1956 too.