Quote:
Originally Posted by
Digmenow 
•When having it altered, is waist suppression a good thing or should it be more sack like?
•Do I button it to allow the tailor adjust it to my body or is there simply no way to have an OTR (thrifted ) jacket drape properly if left unbuttoned?
The odds that you will find a sack tuxedo these days seems pretty remote to me. When I think of "waist suppression," I think of something a bit more complex than taking in the sides. The degree to which you might have it done depends on (1) what is consistent with the style of the jacket, (2) your own physique and aesthetic objectives, (3) what is physically possible concerning what can be altered, and (4) the skill the tailor has and the degree of work he is willing to put into your alterations.
Alterations done to a dinner jacket are no different in concept or process than alterations done to a regular jacket. A good tailor will see how it fits both fastened and not.
My advice would be not to worry too much about these things if you are thrifting. The point of thrifiting would seem to be economy, and economy is usually inconsistent with getting something to fit really well. So, why worry about it? Just get the basic alterations that you need and then go have fun.
Quote:
Originally Posted by
Digmenow 
Please assume a shawl collar/single cloth covered button.
Cummerbund or semi-formal vest?
Cummerbunds look better to me with roll/shawl lapel jackets. The rounded shapes are familial, and each shares its orgination in the more personal/informal history of dinner dress, which is the smoking jacket for the shawl lapel version of the dinner jacket, and the sash for the cummerbund.
If you go back before WWI, pretty much every possible combination of features and accessories was present for dinner clothes. You would be wise to not seek inspiration from that period unless your intent is to affect an extremely antique or eccentric style today.