Okay first off what kind of shelter are you using? If its a closed shelter meaning closed rear beak and front then you will get condensation. I use a tyvek ground cloth and a integral designs sil tarp and never have any condensation. However my golite Shang gra la seems to get quite a bit of condensation.
Condensation is sometimes unavoidable if the conditions are right. Floorless shelter or not it dosnt matter, your sleeping on a pad anyways and that is a vapor barrier. When it gets cold, you will get condensation on the outside of your bag as well as in your tent. The only real way to keep condensation down is to keep air flowing. You should have a means to have air flow through the top of the shelter as well as the floor
A bivy is one way to keep your bag dry when theres unavoidable condensation, or to allow you to open up your shelter more. Some bivies will leave you even more wet though because condensation will bild on the inside of the bivy. Im just purchased an event bivy from a fellow rokslide memeber, im hoping that it will breathe well enough.
Another way is to fire up the stove and cook it off Woodstove, canister or liquid fuel. Iv lit my dragonfly and ran it on simmer more than a few times to dry things out.
I also always carry a small pack towel with me. It weighs almost nothing and has alot of uses, including wiping condensation off my bag, tent walls ect.
ventilation is all you really need though, and thats the way to keep it from forming in the first place
I will be using a bug bivy and (hopefully) a ti goat vertex 5, I always had condensation build whenever I slept in my bag in the Marines when using the bivy, but all I had for a pad was a piece of 1/4" foam. Probably why I woke up to a wet bag all the time in the first place.
Why not just get a normal bivy with a bug net over the face. Alot of guys like the ti goat bivies. There not completely water proof, but they breathe or so i read. That might be a good option to help keep your bag dry along with keep bugs away.
I have the bug bivy and like it but it is more for Midwest camping then protection from the elements. It will not increase the rating on a bag as the top is all mesh. Perfect for protecting a bag and not having to use a ground cloth to protect your bag from the ground. I wouldn't use it as a stand alone shelter if weather is coming in or if a tarp isn't close.
I can fit my bag and pad inside mine, again I think it is the best bag protector but not a bivy shelter.
I use a Katabatic Gear Bristlecone bivy that weighs only 9 oz and it works great in my Shangri La. One thing to note is that most ultralight bivys are water resistant enough for a rain shower, but wouldn't perform well in a downpour. They are really only for use in conjunction with a tarp or other shelter.