For the outdoor part (the Concours d'élégance event itself) you dress practically. Snowsuit or padded jacket plus snow boots. Jeans and fur coats are both acceptable. This includes marquee events such as lunches. A formal trouser suit (for men or women) would be impractical. Bear in mind that temperatures can drop well below freezing and you might be outdoors for hours.

The 5* hotels are very traditional and expect women to wear a smart dress (not necessarily evening wear) for dinner even on non-event days - it's the equivalent of 'jacket and tie required' for men. Same for the smarter restaurants. Everywhere is well heated, but not enough to make men uncomfortable in their suits, so sleeves or even jackets are a good choice. If you don't want to wear a dress, anything above jeans/sweats won't get you thrown out, but you will feel underdressed. A couple of elderly dresses I drag out for this sort of occasion are this one and this, both old DvF. They fall at the midpoint, formality wise, of what other women are wearing.

Unless you're going to an event that specifies a cocktail dress code on the invitation, you don't need cocktail wear. You should not assume that 'jacket required' for men means cocktail wear for women - it is less formal than that. If there is such a dress code, a pant suit is not cocktail wear in Switzerland.

If you're dining anywhere lesser, you can dress more practically (a lot of women go for warm pants and a lightweight jacket). Bear in mind that long velvet pants are not practical on wet and salted roads.