No, it was Vermouth. Gin gave the Boss ICED wine, which means it had ice. Since condonsation (or however it's spelt) would take place with ice present in the drink, the fingerprints wouldn't be clear. So it had to be Vermouth, who never mentioned the Boss having ice in the drink. The method to kill the Boss was most likely that Vermouth inserted poison with a thing you use to draw blood from the vein (don't remember the name... -.-; ) through the cork before she gave the wine to the victim. That is, if it was closed. If it was open, the she just stuck some poison in there through the hole. Them she walked in, gave the wine to the victim, and let him drink it. Boom, dead.
But I still wonder why the fingerprints were clear... The water droplets from condonsation should still be there if they went in, one after another. And wouldn't Gin perfer a gun? Was the bottle open or closed? You did miss a few facts...