I too lost weight when I quit smoking. There are a few reasons for this I believe.
1. Most people satisfy their hand to mouth habit by eating after they quit smoking. Since vaping still keeps the hands active, it satisfies that urge.
2. With all of the sweet flavored vapes available, I haven't had the need to consume much else that is sweet. I used to always be drinking soda when I smoked, but if I have soda now with a sweet vape, it's too sweet so I opt for water or tea.
3. Due to the increase in water, my system remains hydrated and relatively toxin free. Also, people tend to feel thirst before they feel hunger and often mistake the two. Drinking a glass of water when you feel "hungry" will often suppress the appetite until real hunger steps in.
4. Because I can breathe and don't feel like crap every morning, I'm a lot more active and therefore burn a lot more calories.
5. Since my tastebuds have healed, I can taste the actual flavor of food and don't require overly processed highly salted or sweetened foods keeping my diet more healthy overall.
I can't blame it all on quitting since I haven't been working much the last 4 months. I do HVAC and this winter has been too mild so the heaters just aren't breaking 
