Marco Rubio, a U.S. Senator from Florida, is not constitutionally certified to be Vice President. The role of Vice President is determined by the Electoral College and the Constitution.
Picking Rubio would be problematic from an elector standpoint. Constitutionally electors are forbidden to vote for a Prez & VPrez from the same state. They could either vote for Trump or Cruz but not both. In a close electoral count, it could be a problem. Conceivably you could end up with a Prez & VPrez from opposing parties.