I dont think a bivy sack would make much difference, at least inside the tent. The warm, moist air that your body gives off would eventually hit the bivy sack and condense there. So youd still have a damp bag in the morning. Outside, however, a bivy sack might help. Try one such as Mont-Bells U.L. Sleeping Bag Cover ($160; www.montbell.com), which is made with two-ply Gore-Tex.
Whether or not you decide to take action depends on how much of a problem the moisture is causing. Obviously, a wet bag loses insulation, so thats an issue. Plus you have to take time to dry the bag out each morning before stuffing it away. But, unless the moisture is more than a nuisance, Id be inclined to just let it go.
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Filed To: Sleeping Bags