This decision was actually made at IAG (International Airlines Group) which is the parent company of Vueling, British Airways, Iberia etc... not Vueling per se. The main motivation behind it is the price drop on the 737 MAX line. I'm surprised that switching to Boeing it's even worth it considering given these companies mostly fly Airbus planes so their pilots, supply chains for maintenance etc will need an overhaul
most low cost airlines are on 737 actually. I guess the strategy for Vuelling is definitely to go more after ryanair and other holiday carriers.
Some MBA types might also see the opportunity to lay off experienced expensive Airbus pilots and hire new cheaper Boeing pilots, classic strategic gambling, not sure those things ever pay off.
