warm vest or sweater?

John, take your rainproof jacket and shop for a cashmere sweater. They are warm and not bulky and lightweight. Get a really nice color and you'll look great, to boot!

You might consider some Merino wool shirt. I just bought a 150 weight top for my hiking trip in October in France. It was cool there last year, and my one Merino (Smartwool) top got a lot of use, so I just purchased a 2nd one from IceBreaker. Merino wool is ideal, it's warm in the cool temps, but breathes and is great in warmer temps too. Plus, the lighterweight ones can be washed and dried over night (if it's not too humid where you are) The merino wool tops will pack compactly and with a bit of layering should be all you need in addition to your rain jacket.

The Land's End ThermaCheck Fleece pullover or jacket isn't bulky at all. Very light weight. Can be worn under your lightweight shell or by itself. In case you hit a real cold spell in September having a Polartec, Capeline or Smartwool undershirt will keep you toasty. It's all about layers when dealing with the cold.

I vote for the Merino wool at most a mid weight. It is also a little dressier should go to a nice restaurant in the evening. We always take layers. The wool sweater under a windbreak should work well for cool temps.

I like to take a lightweight fleece jacket. It weights hardly anything and packs pretty small, but is pretty warm. If it's really cold, I wear a short and long sleeve t-shirt under it and a waterproof jacket over it. That worked for me at the top of the Alps at the end of September.

It sounds like you would really like to take a buildy sweater. Get a vacuum travel bag that you compress the air out of. It will take up very little space in your bag.

Hard to answer without knowing if where you are going and if you expect to have to deal with both rain and cold or just cold. But.... * A wool sweater can get wet. In fact wool is the only natural fabric that will keep you warm when wet. * Odds are when you are wearing the down vest or sweater you will also being wearing under the light rain shell. So you don't really have to worry about looking out of place...most people won't see it. Nor it getting wet unless you are expecting very heavy rains. * I prefer two thinner layers to one thicker. Consider possibly getting a silk or propylene long underwear top. To wear under a shirt on cold days. *Bring a knit winter hat. Takes up almost no space in your luggage and greatly extends your comfort range. Easy to put on and take off as you go inside or out and stick in your pocket.

My warm layers are either a washable merino wool sweater or a lightweight polar fleece. Merino wool is lightweight, warm and dressy. I spotted a good washable merino (in an neutral olive color) at H&M the other day for $40. Fleece come in three weights -100, 200 and 300 - I go for the 100 because it's the least bulky and still warm. Most of the 100 weight are 1/4 zip. If you search hard, you can find a full zip 100 weight. I like the full zip because it makes it easier to regulate your temperature (plus easier to get on and off). You don't say where you're going. If I were going to Rome in September, I'd pack one merino sweater. If I were going to Berlin I'd bring one merino sweater and a fleece (wear both at the same time if needed). Later in the year, I'd bring a heavier wool sweater and a fleece.

I take a v-neck cashmere sweater( see Lands End )...lightweight and warm. Merino wool would also be a good option....
In late September, you're as likely to hit warm weather everywhere as you are to hit cold weather. That was the time of year I last traveled to Europe( from England to Italy), and in addition to the sweater, I had a lightweight rain jacket and lightweight fleece jacket. Plenty for the coldest weather I encountered, which was at the top of Jungfrau in Switz...

One other thing to consider is that if you under pack they sell clothing in Europe. Pack for the what you consider the likely temps to be not the coldest might possibly be. If it winds up being colder than expected you can always buy warm clothing there. My parent likes sweatshirts as souvenirs. And both wear about the same size as me. They understand that when they get a sweatshirt with a picture of the Eiffel Tower or Big Ben printed on it, it sometimes comes slightly used.

Brad, the trip I'm going on is the Eastern side of France, but includes a trip to Chamonix (hopefully with some hiking there). I've been told that up in the Alps it can be cold at altitude even in mid-summer.

To check on historical weather tables, go here: http://www.wunderground.com/tripplanner/index.asp Of course, this is no guarantee... Don't forget that hats, mufflers, and gloves go a LONG way towards keeping you warm, and are lightweight and easy to pack AND easy to shed as necessary. If you're talking about 'hiking' for an hour or two, just a warm vest or sweater under your (waterproof?) shell (to retain heat and block wind!) plus the hat/muffler/gloves should be fine. If you're planning to spend significant time in the cold, then also bring base layers - check rei.com for more info on the types. Good for sleeping in cold rooms, too. Gloves, a hat, and a thin/lightweight base layer will keep you much warmer than just a heavy coat. Think thin, and 'shed-able', layers. And I vote for your rain/wind shell - again! Does it have it's own passport?!?

I've been very cold in central France in September, with only 2 layers. Late September seems much cooler to me than summer. Do take Eileen's suggestion about the base layers which take up no room at all- I like silk more than the poly. You can buy warm clothes in Chamonix- bought a light fleece layering shirt there for about 50-60 $. Temperatures on Mt. Blanc were cold-not like the valley where the hotels are. In July we put on all the clothes we could manage and still weren't sure we'd be warm enough. As it turned out there was rain,we were fogged in and couldn't go up. I do recall that this forum once found ski clothes for rent in Chamonix for one poster.
(BTW, in fashionable Milan last Dec.,down jackets were far more popular than any other jackets including NorthFace fleece, so down would not be out of place at all if you chose to take it.) .

Micro fleece w/ sleeves as one layer,Thermal t shirt and a long waterproof jacket for rain w/ a hood. Take a ski hat in case it's real cold. We were in Chamonix in July 2010 and it snowed. We were very glad we planned for a range of weather. otherwise we would not have gone up the trams.