The Wikipedia article on the Kingdom of Cochin says:
The kingdom of Cochin was the only kingdom in South Asia to be a protectorate of China. The King of Cochin received special treatment, because he had sent tribute since 1411 and later also sent ambassadors to request the patent of investiture and a seal. The Chinese Emperor granted him both requests.
It certainly seems to have been the only state in South Asia accorded that status by the Ming Dynasty.
In the paper The impact of Zheng He's expeditions on Indian Ocean interactions by Tansen Sen (one of the main sources cited by the Wikipedia article), the author states that:
As part of his fifth expedition, which sailed from China in 1417, Zheng He was instructed to confer a seal upon Keyili and enfeoff a mountain in his kingdom as the zhenguo zhi shan 鎮國之山 (“Mountain which protects the country”). The Yongle emperor composed a proclamation that was inscribed on a stone tablet and carried to Cochin by Zheng He. Both of these were rare acts by the Ming court. Only three other polities, Malacca (in 1405), Japan (in 1406) and Brunei (in 1408), received similar privilege.
So, of the four polities granted that status by the Ming Dynasty,only Cochin was located in South Asia.
It should also be said in this context that the western idea of protectorates, as in the relationship between say, Rome and Numidia at the time of the second Punic war, is significantly different from the classical Chinese view of protectorates/tributaries. The latter was generally a semi-metaphysical recognition of the Chinese as a semi-heavenly kingdom, and did not particularly mean anything much politically, in a strict pragmatic sense. This is obvious from the fact that no Chinese soldiers are known to have fought any wars on behalf of the Kingdom of Cochin, for example. Also there were Chinese ambassadors in almost all the Mallu Kingdoms, since at least the 10th century, largely on the back of the importance of the spice trade route. This is not to say that all the relationships between China and such territories were vague and airy. The Chinese did go for cold realpolitik protectorate relationships with the Uighur against the Dzungars, for example. However IMHO, the political relationship between China and Cochin does not accurately translate to the English 'protectorate' though it may come closest to it, and should be understood more from the perspective of the history of Chinese political sentiments.
While not a protectorate, the Pala Empire was an ally of the Tang Dynasty. It's an early example of Chinese involvement in India. It's rise and fall coincides with the later half of the Tang Dynasty. This was during the period of Indian history called the Tripartite Struggle. Central India was contested by the Western Pratiharas, the Eastern Palas, and the Rashtrakutas in the south. It was the only Bengalese Empire that I know of that conquered Northern India.
