Some theories from an un-qualified passer by:
Since starting vaping 2 weeks ago I have noticed anxiety related ailments when I vape. These include pains in my upper back and the feeling of being short of breath. I have had mild anxiety issues in the past and I can recognize the symptoms in myself now. I have also had the same type of problems when using the patch in the past.
Much of these symptoms were aided when I went to low doses of nic (6mg level).
I don't why I don't get these symtoms from smoking. My theory is that when I am smoking I get the nicotine stimulant with a depressant at the same time. The depressant being one of those 4,000 chemicals or maybe just carbon monoxide. When I am vaping I get all stimulant with no depressant. Similar to drinking 10 cups of coffee for me.
I am still playing with nic doses along with pg/vg trying to make the perfect formula for me, if I find it I will pass it on.
You should probably check out the "smokeless forum". http://www.e-cigarette-forum.com/forum/smokeless-tobacco/