This preliminary work examines the Vietnamese diachronic data to first attempt a glimpse into the inventory of specialized dynamic modal elements in Vietnamese, and to adumbrate a rough chronology of this modal system. It further traces back the syntacto-semantic changes resulting in two distinct instances of the dynamic modal được. I argue that post-verbal được is immediately derived from a resultative marker, and there once existed a pre-verbal được with a pure opportunity reading, which was later reanalyzed as được with a benefactive interpretation. The paper thus provides an explanation to the long-held puzzle regarding the sole post-verbal position of dynamic modal được in Present-Day Vietnamese.