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There is a FAQ "sticky" thread, often linking to resources and threads of interest, as well as an AA Wiki, that might be very sueful to you as you are new here.

The conditions for earning AAdvantage miles on Cathay Pacific are listed here on aa.com. (Vietnam Air is not a oneworld airline, so you would not earn AAdvantage miles on a VN coded flight, even if operated by CX or other oneworld airline; neither would you earn AA miles flying VN with a CX codeshare; and if you flew CX as CX, whether you earned AA miles or not would depend on the fare class purchased. This pretty much is how it works across the oneworld spectrum, unless you are flying an AA codeshare, in which case you earn AA miles as if you were actually flying AA.)