Time to trot out some of the "Inside Baseball" aspects of this "arcane arachnid" issue...
Anything that can foul a tarsal pad on a varroa mite (powered sugar being the most commonly-used substance) can dislodge it, assuming that it has not yet bitten into an adult host bee.
But size of particle matters, as the actual "suction cup" (confusingly dubbed an "ambulacrum" in the seminal paper linked below) is ~50-60 µm in diameter. This implies that anything larger than about 30 µm or smaller than about 5 µm is not going to be effective in stopping the "suction cup" from working. This was why the Dowda Method aka "dump and brush" (very poorly credited to Dowda of FL, where credit is still due) was such a hit-and-miss affair, as one might or might not get significant numbers of particles in the correct size range. The "sugar shake" is at least creating the "fine cloud" of tiny particles within the jar, if one shakes with some vigor.
http://doi.org/10.3896/IBRA.1.50.3.04
So, it may be true that some crystals formed from Oxalic Acid vaporization will form on the tarsal pads of mites, but this cannot possibly be the primary mode of varroa control at work here, as crystal formation is unique to vaporization, and it is well-known that Oxalic dribbling works "just as well" overall as vaporization. So the claim made by the vendor of vaporizers is not unique to oxalic, nor is it a plausible explanation of the primary "varroacidal mode of action" for oxalic acid.
Further, varroa are very likely to be one of the many arachnids that "Secrete a Fluid over Their Adhesive Pads", so the OA crystals on tarsal pads may be a very temporary problem for any one varroa, as oxalic crystals are pretty easy to dissolve in just about any liquid.
https://doi.org/10.1371/journal.pone.0020485
So, how do it all work? (See "Magnets - How Do They Work?" - "Miracles", ICP 2009) I don't think we have any single smoking-gun proximate cause here to support a view that crystals of OA are, in themselves, of any more value to beekeeping than crystals of (selecting at random for a pretty one) Purple Rainbow Fluorite placed atop hives would be of value.
But it can't hurt!
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