Just called Delta and got a good agent who immediately knew what I was asking for, so will add a data point which is really just a newly-dated confirmation of what I've seen elsewhere on FT.
- No space was available on my flight. Per earlier posts/other threads, this matches with the fact that this flight does not show up as an option when I search for an award booking on DL.com.
- Agent volunteered that she had to look for the VS flight #, not the DL codeshare flight #. She also encouraged me to remind other agents of this if/when I call again, as "we've got some new folks who might not know that yet."
- Confirmed that PE>>UC would cost me 25k DL RDM one-way (JFK-LHR). I didn't ask about the luxury tax as I believe that only applies ex-UK.
- She mentioned that VS does sometimes release more space and the UC cabin is nowhere near full, so to call back and check. She also encouraged me to call back as often as I wanted, since there's no way to check for it online.
I guess I'll call back every couple days to keep checking.