I called CX twice and I got two different outcome. One is the same as Xuukgo while the other is the same as Ernest’s. I think bases on the rules, Ernest is correct, but in practice, pax usually check in with the operating carrier as the ticketing carrier most often doesn’t even have counter opened if they don’t have flights that day. And at this case with CX in HKG, I am not sure if they will issue a PP lounge pass on behalf of VN, so just letting the eligible pax using CX J lounge seems more sensible.

VN just changed my flight, and it is VN xxxx operated as CX xxx.