I 2nd the Montbell Inner Down Parka. I think in the High Sierra in June you'd want the jacket, while the vest would be adequate for late July/August. So if you only want to get one, go with the jacket. But you also said you run cold. In men's sizing, the down parka is only 3 ozs heavier than the vest. The inner jacket is only 2 oz heaver for drastically more warmth, if you look at the chart Dan linked to. Down is lighter and warmer than synthetic. The only worry is that getting it wet. As long as you have a shell jacket and some sort of waterproof lining for your pack, it won't get wet.
I also 2nd the rain wrap idea. I have one on order. A small is only 2.7 oz and for keeping my legs dry on nights/days when it might get kind of cool, it's worth the weight for the protection. I don't want to get while while making camp/dinner etc, and then having to go to bed damp.
As far as the 20* bag, my understanding of general sleeping bag temperature ratings is that the rating is generally what is comfortable for a man, and women sleep colder (and you said you sleep cold) so add some degrees onto that. Of course, with a down parka, you could extend that bag's rating somewhat.