If it’s physically too narrow for a small car to fit through, I agree it’s mostly likely not a highway=service
but rather a footway or cycleway.
I can see local exceptions in areas where motorbikes are the main way of getting around, although I can equally believe best tagging could be footway or path with motorcycle=yes
or similar tagging. For example File:Alley in old city (7162772807).jpg - Wikimedia Commons is wide enough to fit a car so it could be service, File:Ngõ Phất Lộc.jpg - Wikimedia Commons is probably not wide enough for a car and personally I would use footway there (disclaimer: I am not a local mapper in Vietnam).