Do I need to repair a slit vapor barrier on fiberglass insulation
If you are installing IC rated lights an are worried about the paper, I would be concerned about the rest of the insulation that surrounds the light.

Typically when I set a "new work" style rough in I cut the drywall bigger than the light rough in by a few inches on either side up to the joists that will support the light. Yes I may cut the paper, but the larger cut in the drywall helps me address the cut and other issues.

"Remodelers" IC rough ins are a different matter.

Once the light is wired in, I add another 4'section or 2 centered over the area where the light will go, after the light is secured in place I cut the paper and the insulation to the light. You can tape the cuts in the paper if you like, and it may improve the performance of the vapor barrier.

The reason why I detail this, is if you have not cut the insulation other than the incidental cut during the time the drywall is cut, I am led to think the insulation is simply pushed up to make way for the light to go in. Doing this raises the insulation above the neighboring pieces and the R-value is diminished drastically in these ares where this happens. This is the meaning behind my opening statement.