About VP selection timing: wiki

Historically, running mates were chosen by political parties in consultation with the principal candidate. In the late 1960s, it became the practice of the principal candidate in presidential elections to announce his preferred choice of running mate at his political party’s national convention. The current practice is for the presumptive nominee of a political party to announce his or her choice for running mate before the national convention which, because of the extensive primary election and caucus system, is becoming increasingly irrelevant.

Wikipedia may have to edit this entry after this GOP cycle.