The temperature range where a vaporizer works is generally between 157°C and 220°C. Below that temperature you're probably not getting any vapor, and above it you risk burning the matter inside of the vaporizer.
The temperatures also vary by vaporizer but a general rule of thumb is that:
160°C to 180°C will produce nice light vapor. It may not be too visible but you should be able to taste something for sure.
181°C to 200°C is the ideal temperature to start off your bowl. It should produce a bit of visible vapor with a lot of flavour.
201°C to 220°C will produce a lot of visible vapor, but you may not get the best flavour at a higher temperature. It's generally a great way to finish off a bowl as you step up the temperature throughout the session.