Actually, nicotine isn't technically a laxative. It is a stimulant, and among other things, it stimulates peristalsis (the movement of your intestinal muscles). If you are constipated, it might be because you are not taking in as much nicotine when you vape as you did when you smoked.
Smoker's bodies get dependent on the peristaltic effects of nicotine, and when they quit or cut back dramatically on smoking, they often experience constipation.
Vape more. Or up your nicotine amount in your e-liquids. Or just drink a LOT more water or iced tea. I'd stay away from chemical laxatives though--they have their own problems and unless you absolutely NEED them for medical reasons, taking them for extended periods of time can cause serious digestive problems...