It's the same company and flights are in codeshare with both, so you might think you buy an Iberia flight, but actually it's operated by Vueling, or viceversa.
There are no non stop flights from Granada (GRX) to Manchester (MAN). You will have to fly either via Madrid or Barcelona.
A quick way to see who's operating the flight (Vueling or Iberia), is to see where if your layover to Manchester is in Madrid (MAD) or Barcelona (BCN).
If you fly from GRX to MAN via BCN, both flights are operated by Vueling, although they are with an IB number too as they are in codeshare with Iberia.
If you fly from GRX to MAN via MAD, both flights are operated by Iberia, although they are with an VY number too as they are in codeshare with Vueling.