I'm not sure how that is normally done, but it is possible that the convention delegates always nominate and vote on the VP. But, also, it's probably just a perfunctory approval of the presidential nominee's choice.
If that is how it works, I guess the GOPe could also play games with the VP pick to weaken Trump. Maybe someone knows for sure the procedures of selecting the VP candidate.
It’s new this year iirc
If Trump walks into the convention he has EVERY right to pick his OWN VP!!!!!