The problem may not even be the rope, as such, if it is double braid construction with Vectran
core and normal polyester or Dacron cover braid. Don't forget that with Vectran you can achieve a much higher load at the clutch (because it won't stretch like the old halyard), so it is possible that the clutch isn't tough enough for the job... so you may need a higher load-specced clutch. Also bear in mind that cluches do wear out, too. The closest metric equivalent to 1/2" would be 12mm, which is less than 1/32" smaller, so I'd be very surprised if a clutch would hold 1/2" but slip on 12mm