The dike surrounding the coast of the Vietnamese Mekong Delta (VMD) helps prevent saltwater intrusion and coastal erosion. In order to preserve these dikes, an area of protective forest is planted outside them, helping to maintain and strengthen them. Therefore, monitoring the change in the area of planted forest outside the dike will help to assess the stability of and possible threats to the coastal area and ecosystem. In this paper, using Sentinel remote sensing images, we propose a new approach; applying the Random Forest technique to assess the changes in the planted forest outside the dike. The experimental results obtained on a typical coastal area of the Vietnamese Mekong Delta will help to clearly identify the threats and the evolution of the coastline through the changes in forest area outside the dike.