Vietnamese thật (truth)-based expressions are prevalent in a 17th-century text and contemporary data, however there has been a lack of in-depth studies of such Sino-Vietnamese expressions in terms of their grammaticalization into discourse markers (DMs). The aim of this study is to identify whether linguistic elements from the same source develop into DMs in a similar manner. To achieve this, I compared diachronic data in the 17th century to contemporary Vietnamese. The historical data includes the first Romanized Vietnamese prose written in 1651, while the contemporary data consists of written and spoken materials collected from a language consultant, internet quotes, and naturally occurring conversations. The main findings of this research are as follows: (1) Thật -related expressions appear to follow the well-attested grammatical path from “true” to “intensive” and further into DMs. (2) The subjective and intersubjective uses of thật forms are distributed complementarily: subjective usage tends to occur in the utterance-final position, while intersubjective usage typically appears in the utterance-initial position. (3) Thật has also combined with other elements to form multiword-unit DMs, indicating that DMs do not always undergo formal reduction. (4) Regarding the borrowing of DMs through language contact, examples from Vietnamese and other languages suggest that lexical elements are initially borrowed as in their original forms and then grammaticalized as DMs in ways that are language-specific yet comparable. These findings provide theoretical contributions to our understanding of the emergence and development of DMs in Vietnamese.