Hi, React
I am allergic to glycerine, so I vape 100% PG.
I have 3 set-ups - Joyetech 510, Kangertech EMUS, and Kangertech mini Protank 3. I have found that I get the most throat irritation with the Protank, the least with the 510 ... and none with the 510 using cartomizers. In other words, in my experience, the more vapour and throat hit a rig delivers, the more irritation it delivers. (I know nothing about variable voltage vapourizers, so I don't know how this would translate to dialing a number on your unit.) I have also found that I reduced the throat irritation by increasing the nicotine, and therefore decreasing the amount of time I spend actually vaping. I am currently using 21 mg nicotine. Since you were a heavy smoker, and are now using low nicotine, you might be "chain-vaping" ... ie, very little rest time between vape sessions.
I found it helpful to read the recommendations on the forum about how to vape ... it is different from how I smoked.
http://www.e-cigarette-forum.com/forum/ecf-library/337017-inhalation-technique-e-cigarette.html
Lastly, the irritation at night could be as much the added-up effect of a day's worth of vaping. When my throat was really bothering me, I found a spoonful of liquid honey helped. Now, with the cartomizers on the 510 I seldom have any throat irritation, but I still feel it when I vape any other device.
Good luck in tracking down what will work for you!