From the Sky report: "...will pursue alternative uses for the London Gatwick short-haul slots."
It doesn't look as though BA is about to gift the LGW market to EZY and WZZ, so some options for IAG are:
Operating a few services as BA mainline with fares to match.
Bringing in an IAG airline to take over some or all the routes, and remember it's not just Vueling, they also now own Air Europa. They could also use Aer Lingus, Iberia Express and/or Iberia mainline.
Leasing the slots out to someone who isn't a challenger, like Jet2.
Putting the BA name on a new franchisee set up to operate the services.