Coastal vegetation is efficient in damping incident waves even in storm events, thus providing valuable protections to coastal communities. However, large uncertainties lie in determining vegetation drag coefficients (CD), which are directly related to the wave damping capacity of a certain vegetated area. One major uncertainty is related to the different methods used in deriving CD. Currently, two methods are available, i.e. the conventional calibration approach and the new direct measurement approach. Comparative studies of these two methods are lacking to reveal their respective strengths and reduce the uncertainty. Additional uncertainty stems from the dependence of CD on flow conditions (i.e. wave-only or wave-current) and indicative parameters, i.e. Reynolds number (Re) and Keulegan-Carpenter number (KC). Recent studies have obtained CD-Re relations for combined wave-current flows, whereas CD-KC relations in such flow condition remain unexplored. Thus, this study conducts a thorough comparison between two existing methods and explores the CD-KC relations in combined wave-current flows. By a unique revisiting procedure, we show that CD derived by the direct measurement approach have a better overall performance in reproducing both acting force and the resulting wave dissipation. Therefore, a generic CD-KC relation for both wave-only and wave-current flows is proposed using direct measurement approach. Finally, a detailed comparison of these two approaches are given. The comprehensive method comparison and the obtained new CD-KC relation may lead to improved understanding and modelling of wave-vegetation interaction.