18% would be 180mg/ml, extremely high nicotine content. Too dangerous to vape. Do you mean 1.8% or 18mg/ml? This is a much more reasonable level. If this is what you are vaping, then there are commercial juices that are lower...12mg/ml, 6mg/ml, 3mg/ml, and 0mg/ml. If you DIY your own juice, then you can make it any strength you want. A tip on reducing nic content is to go slowly, in both how much you reduce your nicotine and how long between reductions. Try some 12mg/ml and if you have nic cravings on that, then try making some 15mg/ml by mixing the 18 and 12 in a 50:50 ratio. Stay at the level that controls your cravings for several weeks, maybe even a couple of months before reducing again. Keep this up until you are at the level you desire.
This isn't going to be possible with a closed system like the Vuse. You will need to get refillable system like a pod or a mod and tank.