That is not correct. Vueling flights coded under IB is a common mistake that many FTers made re: lounge access given that Vueling is owned by IB and operate out of Spain as well. Oneworld rule specified that the flights have to be both marketed and operated by a Oneworld carrier. Individual carriers may grant exceptions to their own subsidiary under specific circumstances but that will fall outside of the premise of Oneworld alliance rule.

While the responsibility falls on the traveler to understand his/her options, it has to be said that the rules are confusing enough that it wouldn't be entirely fair to blame it all on the passengers. For what it is worth, the last time I was in SIN, the CX Business Class check-in staff believed that the Qantas Lounge is off limit to CX Business Class pax because that lounge, in her words, was for "Qantas own customers only". You'd think someone working at the Business class check-in of CX would at least know such basic Oneworld rule. But alas, it was apparently too much to ask even though both CX and QF have been part of the same alliance for almost 20 years now.

Needless to say, when I showed up at the Qantas Club @ SIN, the friendly attendant welcomed me in just by scanning my boarding pass. I had my card out just in case because of what I was told at check-in but the QF staff said there was no need.