My best fix - I found the old STP 125 via Wayback Machine, download link is: https://secure-appldnld.apple.com/STP/071-45899-20210526-3fe7359c-0f20-4850-b6ec-da9b197119c2/SafariTechnologyPreview.dmg

Found the fix! Choose your Target, select Build Phases. Expand "Link Binary With Libraries", press the + button Add your Framework (in this case, VisionKit)... and then choose 'Optional' in the status column. Not quite sure why this fixes it, but it does.