I perfectly understand that despite the number of Ryanair aircraft at Bristol reducing from 5 last summer to just 2 this summer, the number of flights will increase. To be precise, it's currently 242 weekly FR flights to/from BRS in August, as said, slightly up on last year.
We all know FR will throw their toys out the pram IF they are unhappy, but on the plus side, look what they will do if they are happy. Take a look at my local airport, East Midlands for instance, or more recently at Manchester, sustained growth in FR traffic.
If we're talking long term, I fail to see how Vueling can be relied on for long term commitment or growth in Cardiff. I think it's fair to assume there's little chance of Vueling opening a base at Cardiff, the few Spanish routes is as far as it's going to go with them realistically.
I don't believe that Ryanair will completely pull out of Cardiff again once it has set up a base or even serves Cardiff without a base but with several routes to other FR bases across Europe. It's very rare for FR to completely close a base airport and although reductions may occur in future spats, Vueling are still very small in volume so I don't think they will ever end up in a worse position.
Also, it isn't really a matter of choice. In today's economic climate, no one can afford to be too picky about whether it's Vueling, Ryanair, easyJet, Jet2 etc. They all offer a similar service in a nutshell and would offer similar routes. If FR express an interest and Cardiff Airport can attract them, then that is good news. They will not completely rule out Ryanair so they can cling onto a few Vueling routes or hope that EasyJet or Jet2 express an interest in the future at some point (the former I think very unlikely due to it's huge presence in Bristol).