Hate to admit it but I'm right there with the OP. Been vaping for just over a year now and my stamina is completely shot. I'm getting worried myself, seeing my dr. in a few weeks for a full work up to see if anything can be determined.
I'm not sure if it is possible but I feel as though I sleep but my body doesn't feel as though I have slept. I know that too much nic late in the day or night will give me insomnia so I cut way down later in the day but I am perpetually exhausted and feel like I'm buried under 100 feet of water no matter what I do.
Do I need to eat better? Yes. Do I need more exercise? Yes. Has it been a brutal winter restricting activity? Yes. Even taking all of this into account I haven't been very active the last few years anyway so the change is extremely evident and relevant. While I can breath more deeply and don't cough like I used to I feel like I am getting less oxygen somehow. Maybe it is how I vape, what I vape, how much I vape, but it is disconcerting at the very least.
While visiting your Dr, ask about sleep apnea. My brother in law fit exactly the description you're telling - feeling unrested despite getting what should be "enough" sleep. Apparently, his apnea was preventing him from almost any deep, restful sleep, with only a few minutes a night of REM sleep. they put him on a cpap, and after the learning curve with that (which wasn't too bad) his sleep has improved a great deal, and he can feel rested and energetic again.