I've been thinking about trying the type of vest worn with tails but haven't gotten around to it. Personally, I find a cummerbund a bit too affected and I like having my braces under a vest and out of the way.
If you mean a cotton pique vest with the little "wings" on it, I don't think I would go that route unless you were wearing tails. I'm not quite sure I understand your issues with the shawl collar and the vest. Is the vest front cut too low? Too high? Does the "V" of the vest clash with the softer lines of the shawl lapels? There are lots of nice vests out there. There has to be one that works. I have a white on white silk jacquard vest by Robert Talbott Protocol that I got at Nordstrom. Goes nicely with my Tux. Here are some examples: http://cgi.ebay.com/ws....ry=3001
(just the vest, not the rest of the stuff) http://www.roberttalbott.com/Events/Formal/formal.html
(Not many good pictures of their products on the Talbott site.) http://www.directclothiers.com/default....word=ww
(again, lousy pictures. The pique formal vest in this picture is what I think you shouldn't get. The black satin example is more appropriate here) http://cgi.ebay.com/ws....y=15691
(not a bad price for this vest) Below is a picture (stolen from a different post regarding a 3 piece suit I'm having made) of various vest styles. Are you looking for one of these in particular? Perhaps the V-11 style would fit better with a shawl collar. I've got to believe that Talbott makes something in this style. If not, they offer a custom service (probably rather costly however.) I have to believe that any competent tailor could make you a vest, provided you could find the fabric you like. Kai