Vueling should be an excellent addition for CWL, something (as others have said) to kick-start its recovery.
The pre-selection of a specific seat at a nominal fee will be a popular feature for those who want to take advantage of the facility - quite a lot I would imagine as the scramble for seats is a major complaint of many passengers on certain low-cost airlines.
The inability to pre-book a specific seat is certainly the one thing I dislike about using easyJet (or rather the ramifications sometimes are) and speedy boarding isn't always the answer.