Crawlspace Insulation Upside Down Replacement
Yes, cleaning and preparing for the reinstallation of insulation is important. 

Working in a confined area, like a crawl space, requires special protection and ventilation...be careful or hire it done. 

Installing batt insulation without a paper vapor barrier is preferred, but it sounds like you’re going to install a vapor barrier after the batt insulation is installed. Vapor barriers are to be installed on the “warm” side of the “envelope”, because of the Dew point. 

Yes, the crawl space vents are used to dry out the moisture coming up into the crawl space, but it also dries out the VAPOR coming through the batts and turning into moisture at the Dew Point. 

If the vents are blocked, then a dehumidifier may be required where you live, but the vapor barrier is still installed on the WARM side.