The viscous fingering phenomenon in displacement processes develops due to higher mobility of the injected fluid compared to that of the inhabitant fluid. It causes early breakthrough of the injected fluid and results in reduced sweep efficiency of the process. In addition to viscous fingering, heterogeneity of the porous medium can be a source of instability in displacement processes which has significant interactions with the viscous forces. The coupling between viscous fingering and heterogeneity induced channeling has been deemed to enhance the instabilities by increasing the growth rate of fingers formed inside the high permeable channels. This conclusion is questioned by the present work which investigates the effect of the injection velocity, diffusion coefficient, and width of the layers in a layered medium, on the coupling between the two named sources of instability. Numerical simulations and quantitative analysis of the instabilities show that the flow in heterogeneous media goes through four different regimes (initial diffusion, channeling, lateral dispersion, and viscous fingering) each dominated by a different flow mechanism that results in different growth rates of fingers. Based on these characterizations a method is suggested to design the process with an optimum injection rate such that the coupling between viscous fingering and heterogeneity attenuates the instability and results in improved sweep efficiency. Finding the optimum displacement parameters and improving the efficiency of the process helps to reduce the amount of solvent to be injected in the medium and therefore decreases the environmental effects of the recovery process.