As a supporter of both Trump and Cruz, I have a dog in the fight. But I'm also a pastor and regularly speak in public and have a real feel for those who can do it well and those who can't.
Cruz can nail a speech, with charisma, without notes, and with evangelical authenticity. He is also highly intelligent, highly capable, and able to plan a course of action.
His plans are coming to fruition, and they will give Trump some concerns.
It's exactly the kind of person he would have hired.
There is also no way that Trump benefits from being anyone's VP. If nothing else his age gravitates against it. However, the 40 year old Cruz is at a stage that a successful VP term or two would catapult him into favorite status for either the 2020 or 2024 election.
I have a much higher opinion of him than you apparently do.
Leni