Right arm / hand does not follow perfectly the left one with prop editing + x mirror activated


I’m trying to adapt the body of another character to the armature of Vincent. The most difficult part is to adapt the arms and the hands. I’m trying to do that using the proportional editing,but it is still complicated,also because even if I have activated the X mirror,the right arm + hand don’t follow the left ones. Do u have some suggestions ? Do u know a better tecnique ? Can u give a look at the video below to understand what I mean ?