Vertical heterojunctions of two two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted considerable attention recently. A variety of heterojunctions can be constructed by stacking different TMDs to form fundamental building blocks in different optoelectronic devices such as photodetectors, solar cells and light-emitting diodes. However, these applications are significantly hampered by the challenges in large scale production of van der Waals stacks of atomically thin materials. Here, we demonstrate scalable production of periodic patterns of few-layer WS2, MoS2 and their vertical heterojunction arrays by thermal reduction sulfurization process. In this method, a two-step chemical vapor deposition approach was developed to effectively prevent the phase mixing of TMDs in unpredicted manner, thus affording a well-defined interface between WS2 and MoS2 in vertical dimension. As a result, large scale, periodic arrays of few-layer WS2, MoS2 and their vertical heterojunctions can be produced with desired size and density. Photodetectors based on the as-produced MoS2/WS2 vertical heterojunction arrays were fabricated and a high photoresponsivity of 2.3 AW-1 at excitation wavelength of 450 nm was demonstrated. Flexible photodetector devices using MoS2/WS2 heterojunction arrays were also demonstrated with reasonable signal/noise ratio. The approach in this work is also applicable to other TMD materials, and can open up the possibilities of producing a variety of vertical van der Waals heterojunctions in a large scale towards optoelectronic applications.