Book Vueling as soon as possible. The longer you wait, the more expensive the seats become, so jump on their website ASAP. For Sevilla-Granada, take the bus....large, comfortable air-conditioned coaches, with the travel time about three hours. That's the same as the train, but much cheaper, at 18 Euro pp. There are about 10 buses per day....a couple are NOT direct, so avoid those. You'll get your bus at the San Sebastián bus station, about a 15 minute walk east of the downtown tourist areas. Go to maps.google.com and copy and paste coordinates inside the brackets (37.381539, -5.986288) and that'll bring you right to the bus station's main entrance. Get your tickets when you get to Sevilla, and you should be OK. Just remember, bus seats in Spain are assigned, so look for the seat numbers on your tickets.