Even in cloudy cold weather, sunburn can occur in Vietnam. Wear a sunscreen with at least SPF15 for long hours in the sun—the local preferred SPF for sunscreen product is 31–50. Also consider wearing light, long-sleeve clothing that shields as much of your skin as possible from UV radiation.
In case of sunburn, treat with after sun lotion or a cooling aloe vera gel after taking a cool bath, stay out of the sun, and drink plenty of fluids. Avoid alcohol when sunburned—it causes dehydration.