In the real world, using the real rules, one needs to win a majority of the delegates, whether it be in the primaries or not until the convention.
That is when the race is over.
And it's been that way for many years.
In that gathering, and at all others in recent memory, the vice presidential pick has been affirmed by acclamation. But the current rules would subject a vice presidential choice to the same nominating process as the presidential nominee: If there is more than one candidate for the office, a roll call vote of delegates is taken. However, the rules are often rewritten prior to each convention, so it cannot be known at this stage whether this procedure would be used in an open convention.
Meanwhile, there is no playbook to guide the proceedings.
With delegates being Cruz rich and loyal, it looks like Cruz would be the nominee VP, even if Trump stumbles in with 1237.
Trump would likely NOT (unless the rules changed) be able to get a VP pick through by acclamation.