I'd expect that if it were a result of quitting it would have settled down by now. Nicotine is a stimulant, of course, but it was also a stimulant when you were getting it from cigarettes. Has your pattern of smoking/vaping changed since you switched? If you're vaping more in the late evening than you used to smoke, that might be a factor. If that's it, I like Anna's suggestion of having a lower nic setup for the evening - as long as you don't compensate for the lower nic by vaping even more.
Having some sleep issues of my own, I can safely say that sleep issues are downright weird and can be affected by the strangest things. There are even scenarios where stimulants can make you seem to sleep better and depressants can make your sleep worse (sleep apnea, for one). I also wouldn't be surprised to find out that the some of the psychoactive chemicals in cigarettes affect sleep. I think some of them function as anti-depressants, and depression can really screw with sleep (although I wouldn't worry about that much if you don't have other symptoms of depression).
Whatever's going on, I hope you can sort it out before long. Good luck!