Using a log-style vaporizer, it will be your personal use style that is the primary determinant of "efficacy." Many small hits inhaled very deeply, including inhaling air towards the end, and with some rebreathing, is the most efficient method.
But keep in mind, vaporizer "efficiency" is somewhat illusory -- It's the active components that you exhale that are the inefficient part of the equation. That is why dense, heavy hits are not efficient - much is being exhaled. "Efficiency" is also illusory in the sense that what you don't vaporize will still be in there for you to later either revaporize (say after even further drying and powdering) or simply consume in edible form.