Pagan Kingdom and Khmer empire
Located in the Irrawaddy River valley, the Pagan Kingdom was a good place to farm rice. I wonder how big of a rice eater Anawrahta was. He was their first great ruler, after all. He had conquered lands all around, and united them seamlessly. This would provide the Pagans with trading ports, so the kingdom prospered. Anawrahta and the rulers after him supported Theravada Buddhism. Many temples were erected, and Pagan became a center of Buddhist learning. Pagan would fall after one of their rulers refused to pay tribute to the Mongols and losing an attack against them. While the kingdom survived, it would lose all its power.
Just to the southeast, the Khmer empire was doing about the same. They conquered areas around their existing borders, eventually gaining control of most of the Southeast Asia mainland. Their Indian influence was apparent, as their rulers supported both Hinduism and Buddhism. They are most known for the building of Angkor Wat, a large Hindu temple still here today. They were able to fund such constructs due to their superiority in rice farming, from which they gained a nice amount of money. Still, these projects grew so numerous that it weakened the Khmer. And so this combined with the invasions of outsiders would see the Khmer empire's fall.
This video will give you a better idea of what an achievement it was to build Angkor Wat.
Trading Kingdoms and Vietnam
First we have the kingdom of Sailendra, which relied heavily on trade and agriculture. They practiced Mahayana Buddhism, and are known for great Buddhist works and architecture. In fact, their greatest achievement is considered to be the Buddhist temple at Borobudur.
The thing is massive. But there is a lot more to see.
We then have the Srivijaya, the rich guys. They earned their wealth from overseas trade. They realized this was the case, and did everything to ensure they would keep their trade routes. They accepted both Hinduism and Buddhism, blended with local beliefs. Their capital would become another center for Buddhism. Apparently it was so important that one monk would decide the place should be studied at for one year before going to India. Maybe that was why and Indian kingdom would attack the empire. (I'm kidding.) It would be devastating, though. While Srivijaya would survive, they would lose nearly all control to trade. A Muslim kingdom would take over trade, spreading Islam throughout Southeast Asia.
Vietnam was one of the few places in Southeast Asia influenced mainly by China. They had been ruled by the Han, and so adopted some of the Chinese culture. Some of these were forced, such as the language, clothes, and hairstyles. (Taking fashion police to a whole new level.) Their society was influenced by Confucianism and Daoism, and their government was even a bureaucracy with a civil service system. They accepted Mahayana Buddhism, and allowed themselves to be influenced by Buddhist art and architecture. Despite all of this, the Vietnamese retained their individualism. They still worshiped nature spirits, for example.
Vietnam wanted to have their cake and eat it too. They shrived for independence, trying to rebel whenever China was weakened. They wouldn't succeed at first. But heroes would be born from the failure, such as sisters Trung Trac and Trung Nhi. They would lead the force that pushed China out. China would retake Vietnam, and the sisters committed suicide. But they are revered today, and perhaps they were the ones that inspires Vietnam's success. Finally, after the fall of the Tang in China, they became an independent kingdom. (Then just Dai Viet, northern Vietnam). This is the only story that remains a happy one. The Mongols would attempt to invade, but Vietnam would defeat them.