Booking through Iberia you get the all-inclusive price (Avios and a piece of luggage). Through Vueling you have access to a cheaper tariff without luggage and without Avios earned. Also there are some bugs in Iberia.com, that don't allow some bookings. For example you can book BCN-SOF and BCN-SOF-BCN, but cannot book SOF-BCN and SOF-BCN-SOF. Vueling.com does not have this error.