This paper presents the performance of improved soft Bangkok clay with prefabricated vertical drains (PVDs) combined with embankment preloading (conventional PVD method) and vacuum preloading (vacuum PVD method). The performance was evaluated in terms of settlements and flow parameters using analytical methods and numerical simulations in the ABAQUS software. The horizontal coefficient of consolidation (C_h), the ratio (k_h/k_s) between the horizontal hydraulic conductivity in the undisturbed zone (k_h) and the horizontal hydraulic conductivity in the smeared zone (k_s), and the final settlement (S_f) were back-calculated using the measured data. The sensitivity analysis was performed by varying the values of k_h/k_s. The vacuum PVD method was confirmed to have a higher rate of settlement than the conventional method. In particular, C_h increased from 4C_v to 5C_v, k_h/k_s decreased from 8 to 7, and the consolidation time required to obtain a settlement of 1.30 m decreased from 300 days to 100 days. In addition, the calculated results from both the analytical method and FEM simulations for the conventional PVD agreed with the measured data. However, the results from the vacuum PVD method demonstrated that the FEM simulations yielded more reasonable results compared with the corresponding results obtained from the analytical methods.