My visa is for studying in Thailand. Do I need to apply for a Vietnamese visa in Thailand if I fly from Thailand to Vietnam?

If you have a visa to study in Thailand, you should check whether it is multiple trips. If you don't go back and forth many times, your study visa for returning to Thailand in Vietnam will be useless. You should sign another one in Thailand. If you can go to Vietnam, you can usually go to landing visa, but now Sino-Vietnamese relations are so tense that you should apply for a visa at the local Vietnamese consulate in Thailand. If you don't know your visa for studying in Thailand, you can consult your local consulate in China.