Vaping in general takes time to get used to. It is easier to get too much nic in the beginning than from smoking, and the whole nic-on-demand thing is hard to control in the beginning, as Hellen said. Just finding your correct nic level takes time, and this level can change depending on your needs throughout the day. Sometimes I want higher nic with only a few hits. Sometimes I want to vape more with lower nic. If I can't vape at all, however, and there are certainly situations like that, I find I can go longer without anything than when I was smoking full time. Then it was 2 hours max before I would get serious cravings. Now it can be 3 hours or more.
I can't comment on the 5V thing, however. Haven't tried that yet. 3.7V is fine for me at present.
Be patient, don't overdo it, and enjoy!