Do older women dislike foreigners in VN?

I'm curious what everyones experience is/if there is a reason for this/how to get different result.

When I travel around other parts of Asia like the Philippines or Thailand, older women often have smiles and say hi, even if I don't say it first, etc

In Vietnam, notably in Da Nang where I am now, it seems like older women (50 up) rarely smile when eye contact is made and I smile or say hi or xing chao, even if I am buying something or being polite/friendly/trying have a positive interaction.

Most of them literally just stare at me with empty eyes like I'm an alien without saying hi back, or sell me something with zero effort/expression of any kind (?)

For reference, I'm 30M Canadian. Maybe we are a bit sensitive to this, it is common courtesy to say hi back where I'm from, and to not stare people down with a serious face 😅

I've got the same feedback from foreigners I've met, but am wondering why this is the case and if there is a better way to get them to open up/or be friendly?

Older women usually love me, so I'm keen to polish my strategy to make them smile here too ^{^}