My Montbell thermawrap vest is my go-to piece for all sorts of trips --- it layers well, under and even over other things. Just that plus a windshirt or very light shell are good for me for a surprising range of conditions when I stay active. I'm a big fan.

In terms of it being discontinued --- perhaps look other places. At least Montbell themselves seem to still have size medium in stock:
http://www.montbell.us/products/disp.php?cat_id=2002&p_id=2301171&gen_cd=1