There's a straightforward rule on this: operating airline rules apply for each airline only, unless a specific exception has been noted. So there is such a carve out for Aer Lingus, but not, as far as I know, for Vueling. There is probably no way around this since in other contexts you will see issues with smaller aircraft's abilities to carry bags (e.g. Emerald) so the rule is that the operator's restrictions will apply, and unlike checked baggage there aren't over-riding rules. So if you do want to take 2 cabin bags - and in my experience this will be checked at the check-in desk - one of them has to conform to the under-seat dimensions, 40x30x20cm. VY do have some fares which don't include overhead luggage but these are Flight Light fares and are not combined into BA or IB multi-sector trips.