I've just trialed 2 pieces in 32 degree temps and am very happy with them. I agree with others - layering is the key.
1 - A LS Dri-fit 1/4 zip Ideology brand top from Macy's. I purchased this last Jan. as I was on the way to the airport. I was headed for FL, realized I had not changed out of an old waffle weave thermal layer into a newer, nicer one. I think it was on sale for about $20. I wore it every day in the Florida "winter" over a SS Tee on a birding trip, then wore it nearly every day for the rest of the winter over a LS shirt in Idaho. In May and June I did 2 RS tours - Southern England and Scotland and also wore this nearly every day, at least to start with. It's light, folds up small enough to go in my purse, washed well and dries in a couple of hours. It is an excellent layer over a short or long sleeved Tee.
2 - Costco 32degree puffy vest, about $20, I think. I had been reluctant to try a puffy vest because let's face it, I don't need ANYTHING adding any bulk to my frame, lol! I saw this, ordered my size online and just took it to Yellowstone for a week.
As mentioned I just got back from a Yellowstone trip. Every morning I started out with a SS Lands End cotton/modal tee, the LS Dri-fit and the puffy vest. I was able to be out and about in 32 degree temps watching geysers and wildlife. I'm not sure where you're from but I can tolerate a bit of cold so this was comfortable for me. Topped with a waterproof layer, a neck gaiter, gloves and a warm hat or earband and I could easily go down to near zero.
Now, this last week I was just wearing capri length pants as I knew it would be warm by afternoon but I would have been slightly more comfortable if I'd put on my jeans instead. If it were really cold, I would have put on a base layer (new name for long underwear, lol) under the jeans. My base layers are the Hot Totties brand but there are many.
I take jeans on every RS trip I go on. I get jeans that are a poly/cotton mix so they dry overnight. I take the same SS Tee shirts and 1 LS Lands End Heat Crew (sold as a base layer but I size up and wear them for my regular shirts all winter).
I'm also planning to do the Belgium and Holland trip next April! I'll be taking the Dri-fit/puffy vest combo along with my waterproof layer. I'll take my regular jeans plus a pr of Costco travel pants in case it is warmer than I think. I'll take the Hot totties base layer bottoms and a Heat crew top. I'll also take a Land's End cardigan (will fit over the Dri-fit/under the puffy vest and waterproof layer) and 4 SS tee shirts. Really, most of this is what I would normally take on a trip. The only additions are the puffy vest and the HotTotties base layer, both of which squish up pretty small!
I also always wash my jeans in the sink in a 2 gal ziplock. I also wear them 4-5 days (or longer if it's cold) at a time. I generally wash my travel outfit (dark jeans and top plus either the cardie or the 1/4 zip shirt) on the first night - something about wearing jeans for close to 24 hours that makes me not want to re-wear them, lol and then I'll go with 4-5 days between washes for the jeans. Tops I can get 3 days out of unless it is very hot.
I'm looking forward to this tour!