When I first started, vapeing 18 felt like vapeing air. I think the direct comparison between tobacco made 18mg seem weak. Now that I haven't smoked for almost a month, I've managed to drop back down to 18mg which seems plenty strong now.
Another thing is that with the e-cig there's no need to 'smoke the whole thing'. While this reality could potentially increase the net nicotine comsumption, I'd guess that the brain is taking advantage of the ability to normalize nicotine blood levels. With smoking, there might be less overall nicotine involved, but just after having a cigarette the nicotine levels might be higher. To me, being able to normalize nicotine consumption seems like a good way to lower it over time. With tobacco, just a single cigarette causes nicotine levels to jolt up, setting up a cycle that's hard to curb.
I would recommend vapeing more often, and not worrying too much about how much until about a month after the cessation of tobacco use. Once the tobacco feels like history, then set about restructuring the vape habit, and considering lowering the nicotine level before forcing yourself to vape less.
In my experience, I vape more each time I lower the nicotine level, but after a week or so I return to my regular 2ml per day. There is just a touch of effort involved. It was the same when I was smoking tobacco. When I switched from regulars to lights, lights sucked for a week or two, but then regulars tasted too strong. It was the same switching from lights to ultra-lights.