Looking at the photo, I'm pretty sure the crop is tea but I'm not sure. I'd guess that agriculture is the main source of income in this region. I found this on wikipedia.

In comparison with other regions in Vietnam, the Central Highlands has to encounter great difficulties in socio-economic conditions such as the skilled labor shortage, poor infrastructure, possibilities of ethnic-group conflict in a small area and a low standard of living. This area, however, has been blessed with many advantages in natural resources. The region is home to nearly 2 million hectares of fertile basalt, making up for 60% of the national basalt soil, which is very suitable for industrial crops such as coffee, cocoa, pepper, mulberry, and tea.

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