IMO it’s a simple fix for Shopify. The vendor area should be an optional extra and controlled with a ON / OFF switch in the stores settings. If switched off then a 404 page should served for any URLs containing /vendor/
We’ve been lucky with Bing in that we applied some of the changes mentioned previously before they really picked up on it. We spiked at 2.7k Pages, around 2k of them being SPAM related to this issue. And since the changes these pages are gradually being removed from their index.
Google, we saw them pick up and index 367k pages, we made the changes and the not indexed pages hit 2 million. The not indexed pages remains at 2 million, however we are starting to see the indexed pages reducing, we’re now on 300k according to Search Console - I always feel Search Console is delayed so hopefully this number is lower.
Only thing we can do is keep commenting on here and encourage others to do so until a proper fix is put in place.