Hue and Hoian are hardest hit by foehn wind this month. Temperature stays on the high level of 36-37°C (96-98°F) in midday while rain is scarce. It gets terribly dry.
Ho Chi Minh city and Can Tho are in middle of rainy season while Nha Trang is still in good conditions for beach holidays.
note: this forecast for Vietnam weather in July was drawn from July of previous years.
July records a significant drop in the number of international travelers coming to Vietnam. Reservations are not needed but if you go on beach vacation, it is a good idea to book your hotels/resorts beforehand.
Prices also drop considerably this month. You may expect to bargain for low-season discounts.