Couple things at play.
It's a new sensation for your throat so it will take time to get used to. Plus any time you make drastic changes in vapor production you will have to go through it again. Coughing is normal during this time and coughing a lot can result in a sore throat which gets irritated far easier. Both lower power and higher ohm coils can reduce vapor production to make the switch less drastic. I've found that taking a drink of water before taking a hit eases throat irritation a bit.
The flavors themselves could be the culprit. Looking it up, the liquid you mention is a tobacco vanilla. It could be the tobacco, but vanilla has many very different variations so simply looking at the numbers there would be a higher chance of a sensitivity to whatever vanilla is used in the liquid. Plus the manufacturer may have other ingredients in it. The overall flavor may be a tobacco vanilla but it could be a blend of several tobaccos, several vanillas, and some other stuff. Easy way to see if it's the liquid is to try a completely different liquid such as a plain fruit from a different brand and see if it still happens. As an example, I mix my own liquid and there is a flavor called Catalan Cream which I can not use. It ends up being the only thing I can taste no matter how little is used in a recipe and it makes me sick to my stomach. I'm very sensitive to it.
Coil material is a possibility. Try his liquid on your coil and try your liquid on his coil. See what happens.