I am trying to create a system that mirrors a mesh but only the part of the mesh that is covered by the mirror object. The resulting mesh should have new vertices where the “bounds” of the mirror ends.
[EDIT] I need the ability to place multiple mirrors thus mirroring mirrored objects.
How would you go on about instantiating the new mesh and applying correct vertices?
Any thoughts are appreciated! 
If you want to create a mirror or reflection it’s way easier to use a second reflection camera. If you mirror a mesh by hand you would have to invert the winding order of all vertices, reflect the normals and when rendering the object you would need to reflect all light sources as well.
With a reflection camera you render everything as it is but from your reflected camera position.
See this wiki post