Foot rot and jock itch are significant problems. Benadryl is good for allergic histamine release blockade. Good oral hygiene is critical. Good for applying antibiotic ointments to wounds. It may taste horrid but it beats a dose of amoebic dysentery.
Good ones are expensive and almost all are heavy.
Prescription medicines . Bring a full supply of all your prescription medicines. Don't expect to find any particular medication in Vietnam. Good for gluing cuts shut once they've dried out a little. You spray this over cuts and burns and it acts like a flexible bandage.