Back up a minute here. You’re saying your Vietnamese in-laws remarked on how non-Asian their grandchild/niece/cousin appears? Are they happy, dismayed or non-commital about how they feel about this?
Just surprised in a neutral way
In what world can you say “white passing” and have it be neutral? I can’t think of any possible way.
“White passing” might not have been a direct quote. If someone had told me my kids looked a lot like my husband , they would have meant that my kids appeared to be white. Because they looked like me and the only part of their appearance that resembled him was their eyelids. And I’m sure lots of our families were surprised by that - we sure were.
Pretty much this
I have often heard Chinese people say that they find mixed-race white/Chinese people to be attractive. They didn’t use the term “white passing”, though.