Whilst smoking produces toxic smoke and vaping produces vapour that is 95% less harmful than smoking, both can set off fire alarms with relative ease when using your vape juice.
Generally speaking, fire alarms work by shining a constant beam of continuous unbroken light within their sensor. Once smoke enters that chamber and breaks the beam, it then scatters the light across the chamber. Upon scattering, the light beam will then hit the activation sensor which will then sound the alarm.
As vapour will break the light beam in the same way smoke will, it is most likely to activate a smoke alarm in much the same way smoking a cigarette will.
However, unlike cigarette smoke, vapour does tend to dissipate quickly and does not leave any residual smell. As such, it is much more difficult for vaping to be detected in general despite the fact that will activate a smoke alarm just as fast as smoking will.