When we smell a nice plant, we make a blessing on it -- either בורא מיני בשמים, or בורא עשבי בשמים, depending on what kind of plant it is. (Shulchan Aruch, Orach Chaim 216)
Ignoring, for the moment, the question of marijuana's permissibility (spoiler: it's forbidden, according to most opinions); would one inhaling vaporized marijuana make that bracha (presumably בורא מיני בשמים)? Marijuana doesn't smell good, by itself, but it can make a person feel good.
I guess the question can be boiled down to: is the bracha on smell triggered by an olfactory sensation, or by a הנאה (enjoyment) that comes through the sense of smell?