That's one reason it is always a good idea to do before and after mite counts and it really doesn't matter which mite treatment you use. By monitoring you will know how well your treatment worked. It is a lot more work but you will have peace of mind that the treatment worked. When Apistan use to be a common treatment folks would use it spring and fall with very good results. Well then the mites started building resistance to Apistan and it caught a lot of people by surprise. I'm not suggesting they have become resistance to OAV because I doubt they have. But, if you will do some sampling you will know for sure how good your vaporizer is working. If you have screened bottom boards on your hives you can put a sticky board under the hive and you should see a large mite fall for the first few days after your treatment.
I have IPM boards, and did see a lot of mites after the first AOV but after the second, not so many which made me think that the first one got most of them. I did not see a mite all summer until I treated. I looked for them even during inspections, all summer. And to see one attached, after all summer looking, was a surprise. I must have did a semi-failed vaporization, using the torch method. And it is hit and miss as to not burning it too hot, too quick.