Water under the vapour barrier in the attic
Just the fact that you mention a vapor barrier in your attic is the problem. Attics should never have vapor barriers.

Only walls. Next time your at your local home improvement store look at the insulation. You'll see insulation for attics is unfaced and wall insulation has a kraft face, though you can install an unfaced wall insulation if planning on adding a vapor barrier.

Homes will always have moisture and it's through the attic that that moisture needs to escape. That is why your finding the moisture UNDER the barrier.