Vitter’s current term expires in Jan 2017. If he wins the Governorship, he’ll resign in Jan 2016 and appoint his successor to serve until the Nov elections (he’s unlikely to leave his seat until the day he is sworn in because he doesn’t want Jindal to appoint his successor, since he may appoint himself to the vacancy to give himself a leg up).
Well, that’s convenient for Vitter or any senator who wants to run for governor of Louisiana. So, Louisiana will elect its next governor November of 2015, and then Vitter’s senate seat is up November 2016.
And if Vitter doesn’t win the governor’s race, he can still run for reelection as US Senator. That’s good, even though Rubio’s fooling around in the presidential primaries could cost Republicans a Senate seat in Florida.