I believe the minimum corrected vision for VGS Instructors on selection, is 6/12 corrected and uncorrected vision is 6/36, Colour Perception 3 or Colour Perception 4 (with a restricted medical).
Soft contacts are permitted as long as a reserve pair of glasses is with you.
If you then fall outside these limits you have to see an ophthalmic defense specialist before clearance from the CFMO. I'm not sure if this is also permitted on selection.
Although I am open to correction.