It is correct that the nic from vaping is kind of lower and slower.... It builds up in your system over time, you won't get crazy spikes like cigarettes. That said, without the "spikes", I find I can go WAY longer if needed these days without vaping than I ever could while smoking.....
You are also detoxing from countless other products from tobacco, so it's normal to go through some cravings whatever your nicotine level. I have the Nautilus 2, and I like it a great deal. I use 18 m.g. in it and that's what I did to quit, but 15 is not much different, and you can certainly get more lungful of vape on today's devices than in the past, though some people do raise their nicotine quite high to start with... It sounds like you are going great guns though, so hang in there, keep vaping, I'd love to say every DAY gets easier, but it DOES get easier over time. I have very few cravings these days and an actively smoking husband, so I am firmly in the camp that vaping will work.... If you let it.
Keep checking in we are all rooting for you!
Anna