The constitution says a person can't be elected to two full terms as president. Trumps loop hole is that vance could run as president and Trump as vice president, then vance can eventually resign and Trump take over without technically being elected more than twice.
Trump is exactly the type with the sort of gumption to do such a thing. Don't you agree? Trump is the type to make the unthinkable into an option, often an option to half the population