Vapor-solid phase reaction (VSPR) is a two-step process for synthesizing 2D MoS2. In the first step, a precursor film such as molybdenum oxide is grown on a substrate, followed by a sulfurization process at elevated temperature. This process offers a scalable fabrication of wafer-scale film with feasible control in thickness and uniformity. However, the properties of MoS2 films from this VSPR process often suffer from poor electrical properties. The major reason is their polycrystalline (PC) structure with large concentrations of defects and grain boundaries, which are inherited from the amorphous precursor films. Here, we report a new and scalable VSPR process in which epitaxial MoO2 films (grown over a 2-inch wafers) are used as high-quality precursors, which are converted into quasi-single-crystalline (QSC) MoS2. We demonstrate that the field effect mobility of transistors fabricated using a QSC MoS2 channel is almost 35 times larger, compared to a PC MoS2 channel, also better than most previously reported MoS2 films by other two-step MoS2 formation processes. Our process presents a new approach in which the epitaxial growth of the precursor phase can be used to improve 2D semiconductor and device performance.