vaporizing releases about 4 or 5 types of cannabinols (including delta-9-tetrahydracannibinol which causes most of the desired effects).

combustion (smoking) releases about 14 or so types of cannabinols. not all cannabinols cause psychedelic effects though and there are some that are only released through smoking, but not the MAIN one.

combustion destroys a lot of THC in the process of releasing it, leaving less to get you high. vaporizing does not destroy any THC, but it also does not release all of it, so some is stuck in the duff (vaporizer leftovers), though that is why you save the duff and make brownies or oil or something. these things factor into the different high.

another factor to the difference in high is that vaporizing releases only THC and water and all, while combustion causes you to inhale tars and all the burned plant material (hundreds of chemicals), which greatly contributes to the different high. most describe vaporizing as a clearer high, because it is! it is so much more pure than combustion.