It's easy to compare injected heroin bioavailability with vaporizing effectiveness. One example is measuring the amount of morphine in the urine after using both ROA and comparing them.
Vaporizing = a lot of heroin is decomposed. Inhaling vapors of heroin freebase is like ~40% at best from one study but on the other hand other studies give as high numbers as 80%. While smoking heroin in a cigarette is much less. It highly varies, that's the conclusion.
Another point is vaporizing heroin hydrochloride leads to a decomposition of much more heroin than in case of vaporizing heroin freebase. Freebase sold on the street is meant for smoking, people mix it with citric acid or water solution of acetic acid and heat it so the freebase reacts with the acid forming a salt that is much better for injecting. If heroin salt is sold, it's clearly meant for injection.
Inhaling vapors of OxyContin is really stupidity. Even if the pills are crushed and then dissolved in water, filtrated, and water is evaporated. What one ends up with is oxycodone salt with binders that dissolved in water. It's not extracting clear oxycodone from OxyContin.