Flash calculation with VLE considers one homogenous liquid phase and the vapor phase. In case of two only partially miscible liquids (e.g. oils + water) there is an additional equilibrium between the two liquid phases. Thus, the calculation must be performed with the VLLE (vapor-liquid-liquid equilibrium).

VLLE requires a larger computational effort than VLE and chances are that you might find convergence problems. So do not use it unless you have an indication that you really need it (i.e., the liquid phase will probably separate into two liquid phases at the temperatures and pressures of interest).