1854 - 1861
When in camp and off duty, it is socially permissible in the hottest weather to dispense with jacket and even vest; there is no benefit to preserving your period impression and dropping from heat prostration. We have no requirement that you obtain a vest. Etiquette of the period does call for either a jacket or vest to wear even when at leisure, however, and gentlemen of the period - and the men of the Washington Artillery were true gentlemen, after all - observed the conventions and forms of society. These are the two divergent positions.
There is photographic evidence, though, that at least in one camp and in one period of time, some of the men of the Washington Artillery wore only shirts without vest or jacket. As with many rules of etiquette, the men of the Washington Artillery may have gladly observed them at other times and even most of the time, but in camp they were a bit more relaxed in their observance of society's conventions; and particularly in the absence of ladies and in the presence of extreme heat.
Some among us have chosen to have a vest made of the Richmond Grey wool to match their Washington Artillery uniform, but grey or blue and almost any other period color, material, and pattern would be suitable. Some have chosen to get vests made of blue wool so that they generally match the uniform, whether we are Confederate or Federal, and sky blue kersey for a vest would certainly be in keeping with that notion. The style may be military or civilian, which means that the collar may be tall (just like on the uniform jacket) or may have lapels. Buttons do not have to match the uniform unless you choose to have them match.