Singapore is very much the exception to the rule, for example I'm also BA EC Ag, and that means Platinum level on American Airlines, and that does get recognised on board, occasionally for op ups and also lounge access when flying international. For example if you are elegible and flying from Heathrow as a OneWorld elite, you can choose the CX lounge, the AA lounge, or the BA Galleries - most people go for Galleries now but once or twice I went to the CX lounge.
Note HL that SQ and VS are not in an alliance, the SQ J passenger access to the Clubhouse was part of the ownership relationship, not by virtue of alliance.
Skyteam's system is also pretty sorted too in terms of cross airline scheme standardization and recognition. As is *A. So back to SQ, they are one of the primary oddities in that they don't allow partners to redeem miles for J and Suites seats, how they get away with it I don't know, but to me it signals that they're not a full *A member.
Then again other places also put restrictions on lounge access where there is a gulf of a difference between services - I believe BA does this for AA passengers at MIA for example. But these are exceptions.
As for VS, if it joins *A or SkyTeam and stays at LHR then actually there won't be much of a problem - why, because SkyTeam is all out of T4 and *A will be all out of T2 when complete, and is pretty much out of T1 currently except for SQ and SAS, so there really won't be any passengers to flood the Clubhouse.
What will be interesting to see is what level FC Au maps to in the *A scheme - and then whether VS introduces a formal higher tier.
There's a plane at JFK, to fly you back from far away
all those dark and frantic transatlantic miles
