It shouldn't need heat Effie, and as you say it's not advisable. It should dissolve quite readily in cold water so long as there's enough acid to react it.
Mgun, I'm guessing the coke's been washed with bi-carb and there's too much residual bi-carb in the rock you've got and it's cancelling out the vinegar so you're not fully reacting it. Use citric. It's abrasive for one so it will help you grind the rock down in the spoon, and unlike the vinegar you're using you can keep on upping the PH of the solution if needed by adding more. You'll probably get a fizz at first and the water won't clear, so just keep adding a bit more citric a few grains at a time and grind some more till it comes clear. It can take a surprising amount of citric to get there if the crack is poor quality and bulked out with bi-carb, but if you add slowly and don't go mad you should avoid making the shot too acidic. Then you're good to go.