If you have a leak with the lines unplugged, and then no leak with the lines plugged, then that would indicate that it is the lines. If plugging the lines makes no difference, then the lines are probably fine.
You can dive and dye test any suspicious spots on the liner and you can dye test the light conduit. Dye testing the conduit can be tricky, but it is usually doable.
If the conduit is leaking, you can drain below the light and seal the conduit with silicone. Try to only use enough to seal the conduit. Don't pump a lot of silicone into the conduit as that can make it very difficult to replace the light if necessary. Allow the silicone 12 hours to dry before refilling.
Repair the light before sealing the conduit in case you have to replace the light.
If you have a main drain, plug it at the same time, or drain the system, or you could end up siphoning water from the other lines. There might be a hydrostat in the main drain pot, but that's unlikely on a vinyl pool.