Ok, first of all you are lucky cigs by you are only 4 bucks a pack. They are over 7 bucks a pack by me. (The main reason I considered vaping to begin with). Plus you are only at 1/2 pack a day. Not bad. But as you know the costs of cigs are going up. I'm sure they will eventually by you as well.
But the biggest thing I want to point out is that smoking analog cigs is horrible for you! Seriously, Before I started vaping I was going through 2 packs a day at 7 bucks a pop. My habit was 14 bucks a day (98 bucks a week! And I wondered why I was always broke! ) I would wake myself up from coughing in my sleep. Also had that horrible feeling in my chest most mornings from smoking so much. Sometimes it would go away, but it would come back again and again. The worst is feeling your lungs and hearing a whistling while you are breathing. I am only 29 yrs old! My Uncle just died of Emphysema. It got to the point that no matter how much oxygen he was on, he was still suffocating! How awful!! I did not want that to be me. I'm sure you don't either.
Vaping may not be 100% healthy, but it sure does hit the spot if you have the right equipment etc. The main thing is how much better I feel. Just in the matter of 2 days. Regular cigs taste like crap to me. My taste is returning!
So in your case you may not be saving money, but at least you are saving your health and especially your lungs!