Waterproof down bags are typically used in hostile cold climates, where it's subzero weather - they are heavy and typically used with a vapor barrier inside. You do not want vapor barriers in above-freezing temps unless you have nothing else with you - this is why mylar sheets are called "emergency blankets." They will keep you warmER than you would have been, but they will make you sweaty. Not using a vapor barrier in a waterproof sleeping bag will gradually wet out the insulation.

In the Sierra (where I go all the time as well) you need breathable. I have a synthetic quilt with a really good DWR shell that if I use above freezing needs to be dried really well when I get home; the perspiration of the person using it gets on the INSIDE of the shell (it's also somewhat translucent, I can see droplets). Needless to say, the quilt only goes when I know it will be cold.

The ways to combat condensation really have to do with site selection and the shelter you use. Double wall tents are going to have the condensation on the underside of the fly - that's the way they work. Having a well ventilated tent will also help a lot - that's the way tarptents (single wall, lightweight, very good in the Sierra) work. Setting up in a marsh, meadow, grassy area, creek bottom, or similar is going to result in a lot of condensation, whereas getting up on duff (dead pine needles usually) out of canyon bottoms, will decrease the condensation.

We camped in Lyell fork (Yosemite) off Tioga Road on the first night of a JMT outing. My tarp was wet top and bottom the following morning, suspended six feet in the air between two trees, and while I did not get anything in the hammock (including the hammock) dripping wet, I put the quilt out on the rocks in the sunshine which is something you can do most of the time - our Sierra Nevada is quite mild, relatively speaking. Especially in summer. Other than that one time, I have had zero condensation in my hammock setup, generally because I am putting it between young trees - away from water, sometimes over rocks, bear clover, etc. where the tents can't go.

It's actually difficult to wet out a down bag. Droplets on the DWR shell aren't going to do it. Brush 'em off.

If you are truly, honestly concerned - get a Montbell sleeping bag cover. Weighs 6 oz or so, breathable, adds a layer of protection. About $120 most places I've seen it. I know someone who uses it with a tarp and has been happy with it.