The twelfth amendment to the constitution says that no person constitutionally ineligible to the office of president shall be eligible to the office of vice president so no, Cruz could not be elected VP without possible legal questions.
Cruz is a natural-born citizen over 35 who has lived in the U.S. for more than the past fourteen years. He's constitutionally eligible to be president. But since Trump has gone all Birther over him there is no way he could retract his positions and suddenly say Cruz was the man. And I can't imagine any reason why Cruz would want to be Trump's VP.