It’s one of those subtle differences between the West and Vietnam — brandishing your phone lensward isn’t quite as intrusive to your average local. (Whether or not you’re participating in a process of exoticization is another story however, and one that is best left between you, your god and the internet.)
Here is something to consider while contemplating that sweet shot of the napping shoe shiner in the late afternoon sun:
Walking around with a camera is a great way to connect with locals without buying things.
If you don’t speak the language, your options for interacting with non-English speakers will be limited to pointing at noodles and asking, in halting Vietnamese, “how much?” A camera changes the calculus, and gives your brain permission to approach someone that you’d otherwise have no reason to interact with.
And in general, interactions around photo-taking end with smiles on both sides — particularly if you show your model their picture after and ask “Dep khong” (“beautiful or not”)?
If you’re the courteous type, you can ask “Chup hinh duoc khong?” (“is it okay to take a picture”) first, or just smile as you raise the camera to your eye.