Like what others have said earlier, chances are, it's the nic content. Here's a run down on what's going on.
First PG is about as thick as water. VG is as thick as vegetable oil. Things that give throat hit are the quantity of nicotine and PG you're inhaling.
As a smoker, you're used to inhaling gas with minor particles of ash. When you switch to vaping, you'll find the vape is heavier than smoke. Remember the first time you started smoking? you probably coughed a lot, and thought it was harsh...but you continued. It's now like second nature to you now. Well, vaping is a bit denser than what you're used to, so you're going to get that coughing fit again. Especially when you go sub ohm and a high wattage device. The first advice I would say, is to reduce the wattage dramatically. Your kanger subtank came with a 1.2 ohm coil... use that. Lower the wattage down on your istick all the way... then take a vape. If you cough, the next thing you should do is "carb it"... let some air outside the drip tip inhale.... it will lower the density quite a bit.
Now, if it's burning the back of your throat... I would first try lowering the nicotine content. 24mg is extremely high, 18 is high 12 is about normal 6 is light, and anything below is very light. Usually for smokers, I recommend a higher nicotine, but if you can't handle the throat hit, go lower.
As far as switching to VG juice, the Subtank can handle it...but, at a slower pace. Since VG is much thicker than PG, it will take time for the coils to saturate. You can lower the wattage to try and not vaporize as much juice each time you take a drag. Also, lengthen the time between drags. If you want to force more into the wick, do what baditude just suggested and take a couple of "primer puffs" to re-wet the wick. You can speed up the process by covering the air intake, but be careful not to draw too much.
As I typed this, Baditude replied with a much simpler version of what I am suggesting.
Try that, and see if any of those suggestions help. It can get frustrating at first, but it is not unsolvable.