I honestly don't know exactly, I'm sure they were somewhat common. After I saw your question and being you posted a Fort Mahone photo I revisited Adolphus's article on the examination of those photos. If you haven't seen it here's the link, it's a great read and cross section of what a CS soldier looked like at that time.
http://adolphusconfederateuniforms.com/the-confederate-soldier-of-fort-mahone.html He brings up vests. Of the 14 subjects photoed 12 were able to see if they had vests or not. Of those 12, 6 were wearing vests. 4 of those were of military style all of different materials, one was a double breasted civilian style and one there wasn't enough visible to tell for sure. So I think it's a good conclusion it was a half and half whether they had a vest or not and I'd think close to that on style leaning more toward the military style. That said it was still pretty cool weather when those photos were taken. If it were me when the summer campaign season got underway, in order to lighten my load, I think I'd be sending my waistcoat along my great coat, extra blankets, and any other extra baggage I acquired in winter quarters either back home or to a storage area till needed again. Maybe
@Package4 could talk to us both in more detail about waistcoats. It's not really a subject I've paid a ton of attention to.