Trump might surprise you on the issue of VP. If he is serious about saving the country for his grandkids, he might take that VP slot with a candidate he respects. He and Cruz are in agreement on the issues.
Cruz would want his VP to be a vital part of his administration. I think he would turn Trump loose on negotiating with foreign countries on trade and policy.
I suppose it’s possible, but I just can’t imagine Trump taking 2nd seat to anyone. He’s always been his own boss and likes to be in charge. If he doesn’t get the nomination, I think he’d prefer to go back to running his business empire. On the other hand, if someone like Cruz were to agree to give him some real authority regarding certain issues like trade, as you suggest, like Bush did with Cheney for example, then maybe he’d go along. I doubt it, but I guess it’s possible.