I've had this problem recently also, I went to the hospital for it a few weeks back just to make sure I was alright.
I had a chest xray, they watched my heart rate for a while and did a blood test. Go figure nothing happened while I was there, but I still have what I called "Heart Flutters" on and off on certain days.
I told the Dr. about my vaping, also admitted to my obsession with Mountain Dew. He told me to slow down on the nicotine, advised I lower my caffeine intake also. I chilled out on both since then and only experience the flutters on days when I slip up and get carried away again.
I suppose I could describe what it feels like when this happens.
My heartbeat randomly makes itself known, I feel it beating harder, to the point where I think I could see it if I watched my chest. Random increased heart rate, kind of like someone just scared the crap out of me, but... no such thing had happened.
Its no big deal for me though, I just chill out on the nic and caffeine and I'm fine.