Interesting combination. I'm using tonic syrup and soda water instead of tonic water. The tonic and vermouth complement each other nicely bitter-fruity. I used 2oz of gin (because I normally do my negronis at 2:1:1) and it's taking a a back seat beneath the tonic and vermouth. I'll probably play with it to balance them all together to my liking. I feel like more bite is needed on the gin, so I might ease back on the tonic and vermouth a bit.