I'm not a doctor, or a scientist, or whatever, so although I might get a few details mixed up, the basic premise is the same.
Nicotine causes an increase of dopamine in the brain. Dopamine is responsible for activating the reward pathways, which is why we find smoking so pleasurable even though we know it might kill us. That increase is also why people tend to smoke more when doing things like gambling, which also causes an increase in dopamine and is the reason people get addicted gambling.
Anyway, eating also causes production of dopamine, which is why smokers tend to eat so much when they quit smoking, because it really does help the cravings, or at least that part of them. Eating more is why people gain weight of course. As long as you keep your nicotine intake, there should not be any weight gain. For those that have gained weight, there are either other contributing factors, or the act of vaping has cut their nicotine intake enough that they are having cravings and eating more to deal with them. When people say that smoking decreases the appetite, what they really mean is that the nicotine we get from smoking decreases the desire to over eat.
So this long answer is to say the short answer: If you keep your nicotine intake, you should not gain weight.