Is the rest of your rig traditional? I see you're doing peak lapels, what about buttons, vents, pockets? I have a peak lapel tux and cummerbund because I already had the cummerbund and didn't want to splurge for a traditional vest. Especially when it won't be seen anyway. If I had time and the funds I'd opt for a vest, but if you're pressed for time I don't see anything wrong with a cummerbund. Better than no waist covering at all.