How do vests fit into your 3 season layering system

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i like the montbell inner snap neck vest. it is 3.4 oz in size small. dont expect to layer below it, a very snug fit to begin with. i could size up to the medium, i suppose.

usually i will bring this when i expect summer nights below 50°, which is not uncommon where i hike/camp even in AZ. i'll wear it over a lightweight smartwool merino longsleeve and if i expect below 40° i'll add the HH windbreaker that i brought down to 4oz by cutting out the elastic waist.

I usually bring my REI Gossamer vest. Primaloft, light, comfy and packs down nice and small. T-shirt, Marmot ion windshirt, gossamer vest, WT Superlight Alpinist jkt. (waiting for funds to buy an essence)

I find that the Montbell thermawrap vest adds just the right amount of warmth under (or over on rest stops) my wind jacket in temperatures in the 40s and 50s. I think (but haven't tested yet) it would also be just enough for those nights when my summerlite is pushed to it's limits. Just keeping my core warm makes a huge difference to me. I never understood the value of vests until I tried one. My large is ~5oz. I have the down inner parka at 8.5oz for when it's colder. The MB vest fits really snug and I can wear it under either my parka or alpine light jacket just fine. Of course, I have to get the MB jackets a little big anyways because I'm between size and the mediums are just a bit too tight/short. Because I don't spend a lot of time in single digit or sub-zero temps now that I don't live in Chicago, I can't justify a super warm down jacket. I find the triple layer of vest, parka and jacket suits me fine if it gets really cold.

I use a Montbell Down Snap Neck Vest (medium) layered over a Mountain Hardwear MicroChill fleece (size small 7 ounces) for use in the Olympics during summer higher elevation hikes.

I have a R2 and micropuff pullover vest.
The R2 lives in the bottom of my pack and is extra insulation for summer/spring/fall climbs where I will be wearing a R1 for most of the day but might need a bit of a booster other then my belay coat.
The micropuff is harder to use. I bring it a lot in winter as a simaler piece to the R2 in summer. Also nice for craiging.

I prefer a WM Flight vest under a Patagonia Houdini.

I use a 200 weight fleece vest for cool summer nights.

It Could help if I wore it at night when the temps dipped into the 20s and my 30F WM Megalite got chilly. If I'm in the Rockies or Sierras in summer I always take it and lightweight poly long johns.

Eric

I hike in my R2 fleece vest a lot. Not bulky, slim cut works well even under trim MB windshirt, good high neck. What I like is that the full zip of a vest really gives you a big adjustable range of warmth, compared to a thinner half zip long sleeved top. My main beef with the R2 (and fleece vests in general) are the superfluous hand pockets. Marmot does a Powerstretch vest with a single chest pocket which I'd like to try one day.

In camp I use a Skaha Plus Vest in 0.8 Quantum, because it's a really lightweight way to add an insulated + windproof hood to my quilt, plus adding a lot of torso warmth, while skipping on the arms which are more prone to wear and tear.

I used to a Patagonia Nine Trails windproof vest + short sleeved baselayer for bushwhacking, when a long sleeved windshirt would get torn up on bush. Worked pretty well, but not ultralight.