Do I need to repair a slit vapor barrier on fiberglass insulation

Moist air is condensing (barring any oughtright leaks) It is most likely due to air leakage from house to attic:

• Ceiling to attic is leaking moist air, condensing. Solution: air sealing of all penetrations (light fixtures, ducting, electrical, gaps in construction)
• Insufficient insulation, combined with air leakage allows condensation to occur. Vapor barrier (VB) insulation (batt with paper facing) should NOT be placed OVER any existing insulation, creating a trap. If used, the VB is placed against the heated side in winter. Do not use poly for a vapor barrier. The assembly must be able to dry in either direction, should condensation occur.

Update RE can lights Recessed cans are notorious for leakage. If they are IC rated (designed for direct insulation contact) they can be air sealed with a ASTME 136 rated firecaulk directly. If they are non-IC, a XPS box can be built that meets the clearance requirements of the fixture (insulation must be a minimum of 3" around a non-IC fixture (NEC 410.66)) Many commercial products exist for covering cans.

Video: Airsealing recessed lighting by Green Homes America

I've seen this condition develop when a bath vent fan was dumping air directly into the attic and it saturated the insulation and it grew mold down through the drywall.

It depends on where you live and how you plan on attaching everything. For interior insulation you want the vapor barrier on the warm side so the colder regions would always do the interior facing and some warmer climates would do the exterior side of the insulation. The underlying rule of where would the moisture most likely be trapped still applies but it typically biased because of installation, how you plan on attaching everything. For interior walls, no vapor barrier is recommended if you use a full fill spray foam because it is a close cell foam (over 2lb density like most spray foam) and is meant to be a tight squeeze unlike most insulation and stops airflow instead of slowing it down. The same can be true for rigid foam especially for outside insulation. If you plan to use contact adhesive, liquid nails, etc glue to hold the rigid board to the house then you'll be essentially stopping all the air flow and that side would not be a candidate for a vapor barrier, additionally if you are gluing the siding on, the same rule would apply.

To sum up: If you are in a cold climate, even seasonally, it is a good idea to put a vapor barrier on the outside of the outdoor insulation as long as you are not gluing the next layer onto the insulation. This will help the trapped moisture wick out better.