Current vaporizers available work in one of two ways:
Conduction vaporizers are usually considered inferior as only the part of the herb in contact with the hot surface will get heated. This means that a large proportion will NOT get hot enough for the essential oil to evaporate fast enough to be effective. To get round this rather large design flaw, the temperature is increased. While heating the rest of the herb up high enough can be achieved through this, the part in contact with the hot surface will be overheated (occasionally to the point of combustion) thus negating the point of vaporising in the first place.
These are commonly referred to as 'soldering-iron' vaporizers because most models appear to use a soldering iron element with a bowl on top to put the herb in. If the vaporiser look like a heated stick in a jar, its probably one of these.
Convection vaporizers are much better because heated air can flow all the way around all parts of the herb. This ensures equal heating, and consequently more complete vaporisation. As long as hot air flows through the herb and there are essential oils left to vaporise, it will continue giving off vapor.
Convection vaporizers are more expensive because they are usually thermostatically controlled. It is also more difficult to heat a flow of air to a constant temperature because there are more variables eg. speed of air, air temperature before being heated, etc.
Some vaporizers appear to use radiant heat but most of the effect is caused by convection. Radiant heat is difficult to control without actually measuring the surface temperature of the herb which in itself requires expensive IR thermometry. That is not to say it cannot be done.