I should have been clearer: I think your pants will be perfect. I want a pair. Bring a hard shell to carry in case of emergency. I think they will only be too warm on day in which you are stripped to your base layer on top and still sweating, and there is just isn't that much chance of that in Vt in Dec.
Mansfied from the east is dominated by the ski resort, so I think it is much nicer from the west (but I live over here). Mansfield involves several hundred feet of above treeline exposure, which is almost always wind swept and icy. I haven't done them in many years, but the main eastern approaches are the long trail from the north and hell brook. Both are steep and both involve exposure up top, so not something I would recommend.
My first choice recommendation is to do Mt Hunger. A very nice peak, 2000+ feet of climbing, a nice trail that will be packed for sure and is very popular, but well worth it. Minimal exposure on top since you can get back to treeline in a few minutes, but a very nice summit.
Camel's Hump is the nicest peak in Vt and one of the best in the east. All the trails on CH are beautiful, so except for the bamforth ridge, any would be be great. The obvious choice is the monroe trail from the duxbury trail head.
If you do Hunger and Camels Hump you are sampling some of the very best peaks in Vt (although very, very popular peaks). Each will take a half day, so you would have to pick if you only one day, since we have about 9 hours of daylight right now. For the after hike recovery, I recommend the alchemist brew pub in Waterbury.
The NE hiking forum is called views from the top, and there will be tons of info and trail conditions there:
http://www.viewsfromthetop.com/forums/index.php?