Neat!
In case of WooCommerce the popularity is determined by the “total_sales” custom field. If you use that, it should work. However using it as the primary ordering field is probably not the best idea, as other items matching the query with higher sales numbers will be displayed first – which are not neccessarily the best matches.
The secondary ordering could work though, but that is obviously applied only after the primary and only for matches having the same exact relevance. It is very unlikely to have matches with the same relevance when the keywords are very short (like “a” in “vitamin a”), so you may not see much difference in the final results order.