Let me see, nicotine, like caffeine and a a few other common things are mild laxatives. We drink more fluids when we vape, good for getting softer stools. We removed the other chemicals in smoke which have who knows what effects. Yep early on it can easily make the laxative effect more noticeable. Did for me too but, that goes away after a few months as your body gets used to the lack of a few thousand chemicals you used to get in smoke and, adjusts to you drinking more fluids.
Change juice, if that changes it, then adjust there as much as you can and, be patient with your body as it cleans out all the analog leftovers and gets used to actually being properly hydrated.