Text readability is a measure of how easy or difficult it is to read a text. This readability factor plays a crucial role in the processes of drafting and comprehending the texts, affecting the choice of proper texts for reading. Studies on the readability of text have started since the late nineteenth century and there have been many practical applications. However, these studies are mainly performed in English and other popular languages. In Vietnamese, the study of the text readability is still relatively untapped and has only received attention in recent years in the process of improving the curriculum and teaching methods. Recent studies on the readability of text in Vietnamese language are still limited, the main reason was largely due to the lack of text resources, which are corpora graded accordingly to difficulty levels. Therefore, in this study, we focused on building a corpus for assessing the readability of Vietnamese texts in the literature domain through the process of collecting, processing and evaluating documents. The result is that we have built up a corpus of 1,825 Vietnamese texts, divided into four levels of difficulty (Very easy, Easy, Medium and Difficult). Experiments with the existing Vietnamese readability assessment methods show that the built corpus is reliable and usable for further research on the text readability.