Both countries eat with chopsticks, so table manners overlap closely. One point reverses completely.
The shared taboos
Standing chopsticks upright in a bowl of rice. Japanese calls this tate-bashi or hotoke-bashi, avoided because it is the form of the rice bowl offered beside the body of someone who has just died.
Vietnamese avoids it for the same reason: a bowl of rice with chopsticks standing in it is the bowl placed on the ancestral altar. The reasoning matches exactly.
Passing food from chopstick to chopstick is a strong taboo in Japan — hashi-watashi — because it echoes the transfer of bones with chopsticks during the ritual after cremation.
Where they reverse
Here the two diverge. At a Vietnamese table, using your chopsticks to place food in someone else's bowl is hospitality. A younger person serves an elder; a host serves a guest. The word is gắp cho.
In Japan the same gesture risks giving offence. Food is taken from shared dishes with serving chopsticks (toribashi), and touching another person's portion with chopsticks that have been in your mouth is avoided.
A Vietnamese employee kindly serving food into a Japanese manager's bowl, and the manager freezing — this happens often. Neither is wrong. The premises differ.
Eating audibly
Sound reverses too. In Japan, slurping soba or ramen is accepted. In Vietnam, as in many countries, making noise while eating is considered impolite.
Telling a Vietnamese guest that slurping is fine does not remove the discomfort. There is no need to insist.
What is said before eating
Japan says itadakimasu before the meal and gochisōsama after, everyone together.
In Vietnam, the youngest at the table invites each elder individually: con mời bố mẹ ăn cơm, addressed person by person in order of seniority.
Different forms, same idea: something is said before anyone begins.