The controllable large-area growth of single-crystal vertical heterostructures based on 2D transition metal dichalcogenides (TMDs) remains a challenge. Here, large-area vertical MoS2/WS2 heterostructures are synthesized using single-step confined-space chemical vapor epitaxy. The heterostructures can evolve into two different kinds by switching the H2 flow on and off: MoS2/WS2 heterostructures with multiple WS2 domains can be achieved without introducing the H2 flow due to the numerous nucleation centers on the bottom MoS2 monolayer during the transition stage between the MoS2 and WS2 monolayer growth. In contrast, isolated MoS2/WS2 heterostructures with single WS2 domain can be obtained with introducing the H2 flow due to the reduced nucleation centers on the bottom MoS2 monolayer arising from the hydrogen etching effect. Both the two kinds of the vertical MoS2/WS2 heterostructures feature high quality. The photodetectors based on the isolated MoS2/WS2 heterostructures exhibit a high responsivity of 68 mA W−1 and a short response time of 35 ms. This single-step chemical vapor epitaxy can be used to synthesize vertical MoS2/WS2 heterostructures with high production efficiency. The new epitaxial growth approach may open new pathways to fabricate large-area heterostructures made of different 2D TMDs monolayers of interest to electronics, optoelectronics, and other applications.