A shaken open hand in Vietnam means it's all gone - empty. Like I rattled the can and it's definitely finished. But an American would think it's a basketball or globe spin thing going on.
If someone picks their nose slightly and flicks it it's just means get lost. It's not a drama to them because running noses and boogers aren't as common in a tropical country. They don't get plague vibes from everything booger related. Usually it just means your joke sucks.
An intro nodding forward is saying hello with respect, almost like a fist bump with Americans. Folding your arms for older people and slightly bowed nod is double or triple respect. But with most people just return the slight respect.
Use two hands to pass business cards and bow a little or else you're not really considering them as business clients of any renown. This is the same in all of Eastern Asia AFAIK.
Take your shoes off! Huge mistake in some houses.
Don't stick your tongue out or use middle finger. It's viewed as much worse than foreigners expect it to mean to locals. Like you're defaming them or something.
Don't pat people on the head in South East Asia unless you know it's okay with everyone including bystanders. The temple of the body or something to some people.
Some people don't like seeing kissing in public.
Girls with midriffs/exposed bellies used to piss old men off. Don't know if that's still a thing.
Seems feasible considering every attempt to discuss using more Fediverse just gets ignored. Like Peertube is sitting right there, but everyone that joined Mastodon instances is sharing YouTube links.
But people are like this in general, so it might be just predictable behaviour.
Friendi.ca is a Facebook alternative, but I don't want to leave anonymity again.
Pixelfed is a good Instagram alternative.