@ Mach:
As I told you in my previous posts, LCCs already have fuel surcharges and many others. And of course, if Clickair and Vueling ever merged, they would also increase the price of the tickets.
@ wince:
For sure they will keep buying LCC tickets. To me, Vueling is not a particularly cheap airline, and people keep buying tickets to them, even when their fares are above the ones of Iberia or Spanair. Keep in mind that:
1.) Sometimes you cannot choose between a LCC or a legacy. For instance, here in Barcelona, the customers that like Iberia & Oneworld (as me) cannot choose Iberia or Clickair: they are directly transferred to Clickair. The same happens in many airports. If you live in, let's say, Zaragoza and you want to travel to Europe, either you take Ryanair or you have to take Air Nostrum to Madrid (OK, Air Nostrum also has international destinations, but they are limited). In some airports, the LCCs are the real "gate" to the world (considering that many persons don't like stops in a hub).
2) Some segments don't even try legacy carriers. For instance, college students. If they want to go to wherever, the first place they go is Ryanair, EasyJet, Vueling, etc., but I'm sure that (almost) nodoby types in the browser Iberia.com or Spanair.com or BritishAirways.com or whatever. There is a strong commonplace: Ryanair might be expensive, but any other will be even more expensive. There are many many persons used to travel with LCCs that don't even try legacy carriers. I always find legacy carriers cheaper or more or less the same, but not everybody is open to buy a non-LCC ticket.