The basic question is how to write a simple script that copies one vertex position to another vertex and mirrors one axis, as well as copying and mirroring all vertex weights if possible.
Two selected vertices should be mirrored, but due to user error, are slightly off and break the mirror symmetry in edit mode.
I have tried using the symmetrize function already in blender, but this breaks weight painting values for the model and leaves duplicate vertices behind. Manually copying every x,y,z position and vertex weight is not efficient at all.