temps for Montbel exlight jacket?

for me … YMMV …

I'm pretty warm-blooded and was comfortable down to the low 20's in a setup similar to yours but without the vest or the windshell. I'd say you should be good to 20 with the vest and the extra outer layer.

long sleeve base layer + ex-light + windshirt down to freezing- my metabolism is little (maybe more than a little?) on the warm side

at freezing w/ this combo you should be comfortable around camp before turning in, sitting for hours I'd venture you'd start chilling

a light fleece vest should add a few degrees to the above

If it's expected to be much below freezing I won't take the EX Light for camp and sleep system use – not warm enough. It replaces fleece and is intended as a midlayer.

Richard

Wearing all of that you could probably get into the 20's.

There were many reviews of these jackets which you can read if you are a member.

And 6 reader reviews that are free.

Maybe 20s F.

IMO, these 2oz fill jackets pair well with a 30º bag, maybe even a 20º bag. But they aren't winter pieces unless you're layering several insulating garments.