My friend, and business partner (Consortium Cabernet) Dan Petroski makes his own vermouth through his label Massican. He decided to run some tests on a FOSS machine of the residual sugar levels of many of the popular vermouth and bitters. With his permission I am posting the results as they are quite eye opening to the amount sugar found in many a sommelier's favorite mixer.
All RS is listed in grams per liter
Noilly Prat Extra Dry - 24g/l
Martini & Rossi Extra Dry - 30 g/l
Martini & Rossi Rosso - 155 g/l
Dolin Dry - 29 g/l
Dolin Rouge - 146 g/l
Carpano Bianco - 178 g/l
Carpano Antica - 194 g/l
Lillet - 98 g/l
Aperol - 263 g/l
Campari - 253 g/l
The Aperol and Campari numbers are the ones that strike me as holy crap!!! Those numbers mean every bottle is over a quarter sugar given the bottle size. I'd probably be scared to do this test with Chartreuse as my consumption habits would probably decrease if I knew the g/l sugar.