I have experienced a few mornings where the OCF pad was a bit damp, ironically these were not really cold nights. If you read my Multimat review you'll see pictures of frozen condensate on that pad in sub-zero conditions after a night in the UC. In fact, there was enough ice there that the multimat froze to the OCF pad causing some damage to the OCF when I separated the two. It might have worked better if I had the nylon side up instead of down.
On the nights where I've used down jacket and/or sleeping bag in addition to the OCF pad in the UC I have never seen enough condensate to cause a problem with moisture in the insulation.
I generally don't go out for more than 3 consecutive days in winter, so I can't testify to truly long-term impact.
What my experience to-date would indicate is that if you use the SB as a top-layer VB, condensation will not be the biggest problem, rather getting enough insulation well-distributed and suspended properly in the UC is. The UC is not an ideal insulation suspension system: it is slippery and elastic. It works, I've put lots of stuff in there, but it doesn't like to stay out at the ends. Thats why the OCF has elastics.