The nic could be a possiblity for the headaches, but if you haven't really started detox yet, the 18 mg could be just right for you over the next couple of weeks. The vg helps promote vapor. I personally started with the same set up you have, juice and all, but upped the vg to help me keep from pulling so hard. For ME, that was the cause of the headaches . . . I was working too hard trying to find the same "smoking" experience. It's my understanding that although vg increases vapor, it can reduce flavor. I didn't find that to be the case, but I can't dispute it either. The vapor was more important for ME. As I played around with juices I found the biggest indication that I was getting too much nic was quiziness, not headache, but everyone reacts to change differently I'm sure.
There are many threads that talk about the pg/vg ratios, nic strength, detox symptoms, etc., so as I recommend to all . . . read, read, read.
Congratulations on making the switch and WELCOME!!