It'll be the first planetary launch from Vandenberg. InSight is a very small spacecraft and the Atlas V has power to spare. To relieve congestion out of Florida and because they could, NASA planned a launch from Vandenberg.

The main difference between Vandenberg and Kennedy launches is that Kennedy launches to the east and Vandenberg launches to the south. Typically, Kennedy launches produce orbits closer to the equator and Vandenberg produces polar orbits. In most planetary launches, the rocket delivers the spacecraft into Earth orbit, then there's a short coast phase, and then the upper stage restarts to take the spacecraft out of Earth orbit and onto cruise. Kennedy's eastward launches take advantage of Earth's rotation to help throw payloads into orbit, so if you're pushing the limits of your rocket, it's nice to have that help. But once your spacecraft is in any Earth orbit, it takes the same amount of thrust to move from that Earth orbit to a Mars transfer orbit regardless of the orbit inclination. (For a little more info, read this tweet thread by Mark Wallace, who designed the trajectory.)

The launch vehicle is an Atlas V 401. InSight's will be the 62nd launch of the Atlas V. "401" signifies that the payload fairing is the smaller option (4 meters in diameter); there are no extra strap-on boosters attached to the common-core booster; and the Centaur upper stage has a single engine. The MarCO satellites ride inside an aft bulkhead carrier, at the bottom end of the Centaur.

InSight will separate from the launch vehicle within 90 minutes of launch. Its cruise stage solar arrays are already fully deployed at launch. After separation, it will determine its orientation in space and then turn to the correct attitude for communication with Earth. The Deep Space Network radio antennas at Goldstone should be the first to pick up its signal, some time between 0 and 14 minutes after spacecraft separation.