South of Vietnam has two seasons: dry season and raining season. During dry season (typically from November to May) the weather can be very hot, be sure to drink enough water and to protect you against the sun. During raining season (typically from June to October), the rain can be strong, shelter and wait the end of the shower. It usually does not last long.
There are a lot of mosquitoes in the Mekong delta due to the presence of many rivers. Mosquitoes can bring you dengue and malaria, so use frequently mosquito repellent even during the day. If case of high fever lasting more than 3 days, you should consult a doctor.
In the countryside, there are many animals moving freely (dogs, chicken, ducks…), please do not intend to touch them.