Teach me how to layer down. Sizing and Lofting.

Down can be compressed some and still retain it's insulating value. (There's a post by insulation expert Richard Nisely stating that somewhere on this forum.) If moisture isn't a factor you're concerned about and down is your preferred insulation, then get the down vest and down jacket. On the other hand, maybe it would be good to have one down and one synthetic so that you can use one in warmer, moister environments.

For sizing, it's hard to tell by description. I think you'll need to try the jacket on over the vest to know for sure. If it feels a bit restricted, go up a size. For the neck, it probably wouldn't matter too much if you left the outer neck roll unfastened.

Are you looking for active use, or rest/camp use?
That makes a big difference.

Rest/Camp use. I'd like it to be versatile for everything, ideally. Warmest with low weight/bulk. I guess I'm just trying to decide between down and synthetic vest, then whether or not it should be a smaller size. Thanks.

What do you guys use for your inner/mid-layer insulated vest? and did you go with a smaller size?

Why do you want a vest? A heavier jacket will be warmer for the weight.

"Why do you want a vest?"

Vest insulation covers just the torso. The torso is warmer so a given amount of insulation will prevent more heat loss.

And since you move your arms around to do stuff, insulation hinders this – "Michelin Man" effect

At least that works for me down to maybe 20 F.

But a lot of people like jackets so that works too.

Consider a fleece vest to use with a down jacket. My favorite is a Marmot Power Stretch, but an R1, R2, Synchilla, or other quality fleece would do the trick– your choice for loft/weigh/cost. It can be layered with the down with no worries about effecting the loft, sitting close to the body and trapping a good layer of warm air. A fleece vest has good breathability and moisture management qualities and avoids hauling two more layers of shell material. The vest stands well on its own and it is excellent with a windshirt or rain shell, or for sleep. I'm assuming it will be used with long sleeve silkweight or Cap2 base layer top. Add a nice puffy down jacket to the mix and you'll be warm at rest.

Jerry, I ask because there could be so many reasons, and it might be worth reconsidering the system.

I have a WM Flight vest. With 2.5oz (spec) of down, it weighs a little over 6 ounces in a size small. Let's look at a brief combination:
WM Flight Vest —- 2.5oz down, 6.1oz net
Rab Alpine Jacket — 4oz down, – 10.5oz net
Total

Not a very efficient system. The vest can't be worn on the move. They're only "parts" in so far as you could take half on a trip if it weren't so cold. But in cold weather, it's heavy without an extra-durable (or weatherproof) shell.

Stephen, I'm not saying you're wrong or anything, many good ways to do things

If you had a jacket that weighed the same as your vest, it would be all shell fabric, no weight left for down, it wouldn't be nearly as warm as the vest.

Or, another way to think about it, I wear a long sleeve shirt, vest, and jacket, and stay warm down to 20F, my arms aren't uncomfortable. If I took the insulation in the vest, and distributed some of it over my arms, they would be a little warmer but my torso would be colder, plus if I preserved the amount of weight, I'de have to remove some of the insulation to allow weight for the arm shell fabric.

I think that below 20F you definitely need a jacket, but I avoid that condition so I don't care, I'm just talking about warmer temperatures.

"Rest/Camp use. I'd like it to be versatile for everything, ideally. Warmest with low weight/bulk"

Since you do not intend to use it while hiking, I'd suggest something like a Montbell
UL vest or UL "T"(which has partial sleeves for only 1.5 oz more weight) over your base layer, followed by a lightweight down parka. The parka could be Montbell, Rab, or other lightweight parka of your choice. My favorite is Montbell, but that is strictly personal. In combination with a windshirt or WPB shell, you should be good to the mid 20's around camp. Synthetic just doesn't have the best warmth:weight ratio, although it has its uses in wetter climates or if you intend to wear your insulation while on the move, which I don't think is a particularly good idea in most cases. Those who do wear insulation seem to prefer fleece to Primaloft. You might PM Dave Chenault for more input on this subject, as he has a lot of experience in wet weather hiking. A lot of experience, period, for that matter.

Jerry, I agree with you. My main point in all this is asking the OP for their intentions and goals. Down to 20º is very different from down to 0º.