Rice yields vary wildly around the world but Vietnam averages about 6 tonnes per hectare, so you can probably get 300kg of rice from 500m2 of land. So not enough to really live on, although if they are in a very productive region that amount could be doubled.

According to the thread, one of her aunts in the village works the land becuase she moved to the city and she dosen't get any kind of rent from her aunt.