Very happy I found this thread. Booked a Jan-23 VS LAX-LHR PY r/t ticket through DL yesterday and really didn't think I had any viable upgrade options. After reading this thread, though, I checked EF and it showed G inventory for both legs.

Called Delta and had success with first rep I spoke with. She was dubious at first, but when I mentioned the VS G booking class she called the partner desk and confirmed the upgrades were available. I real-time transferred 50K AMEX points to my DL account and $204 in taxes later, I find myself in UC both directions.

Never would have know if not for the experience and expertise in this thread. Many thanks to all OPs!