From what I recall from pharmacology and physiology texts, by the time any aerosol particles (including liquid vapors) small enough in size reach the small sacs (alveoli) in the lungs, deposition on the alveoli membrane and diffusion through it and into the blood stream happens rather quickly and efficiently in a matter of a few seconds. I recall a general rules-of-thumb being that about 90% of particles less than a few microns in diameter are rapidly, in seconds, absorbed when drawn deeply into the lungs and held for at most a few seconds. After that, you may get at best a few percentage points more by holding ones breath. This is advantageous for prescription drug delivery, since most everyone, no matter how sick or inexperienced at consuming drugs by inhalation, can be presumed to be able to inhale something deeply and then exhale.
Keep in mind that a fair amount of inhaled actives not sufficiently deeply inhaled or the vapor particles are too large get deposited on the surface throughout the upper respiratory system. Between the natural upflow of mucus washing out the lungs and swallowing of saliva, essentially all of this ends up in the stomach, where it is less efficiently and slowly absorbed. This is perhaps a reason why some people say they feel stoned from the night before after eating something in the morning (with their digestion system kicking in).
Rather than holding it long, try rebreathing -- exhale the useless (never to be absorbed) vapor in your throat and upper respiratory system, a good portion of your lung capacity, and suck down some fresh air which will end up mixing with the leading stream of vapor you inhaled. And if efficiency of consumption is important, take shorter hits and then suck each down deeply by inhaling regular air.