I just bought a synthetic puffy vest at Target for $3.75 - closeout. It is bulky but warm. This is a good time of year to find end of winter stuff. If you only need a "sitting around camp" jacket or vest, you always have your sleeping bag as back up, so it need not be huge.

I do not take a down jacket ever in the Sierra from May to end of OCtober. I have a down vest that I throw in if weather reports are for colder than average. (in addition to three 100-wt. fleece layers or medium weight wool layers, a light wind jacket, 100-wt fleece long johns and rain gear). FOr very little extra weight, take two hats one regular stocking cap and a balaclava . Another idea is set up tents all facing each other and you can all be in your sleeping bags inside tents and still socialize. Perchance it gets cold, just retire to tents.

The extra layer or down jacket is really more convienent for mornings. Not essential, but really nice. I have seen many early mornings in the Sierra where it was in the mid-20's when I got up. And this includes August. But I was aso above 10,000 feet.