So many thanks for the comments!
We know for sure that Die Vecna Die! takes place in 591 CY not just because the LIving Greyhawk Gazetteer describes its events as "recent," but because in the Greyhawk Players Guide (which is also set in 591 CY), the events had not yet happened.
Citations: The Greyhawk Players Guide, published in 1998, identifies the current date on page 2. Vecna appears on the list of demigods on page 20. The events of Die Vecna Die involve the demigod Vecna briefly becoming a greater deity and ending the book a lesser deity (page 150: "He's free of the prison plane that held him and, despite the liberation of Iuz, still a god." He's described as a lesser deity in the Living Greyhawk Gazetteer, page 186, which notes, "He recently became a lesser god after freeing himself from an extradimensional prison.").
Hmmm... Not sure if I'll add that or not. I don't think it countermands what I wrote, I just wish I owned Die Vecna Die
so I could judge it better. The one year discrepancy is just too minor to sweat here. Especially since the Player's Guide might just describe what players know, not reality. There would be a period in 591 where he was a demigod before becoming known to be a lesser god.
At least one other Planescape accessory does indicate that the current date is after the Summer of Chaos on Krynn. A Guide to the Ethereal Plane (1998) describes Krynn's Border Ethereal on page 16: "Chant is that this world fell under the sway of some chaotic power. No one knows if that's the dark of it, but fact is, a body can't reach Krynn any longer. Many bloods've tried, but they all return rattling their bone-boxes about ether gaps and creatures born of chaos. If that's true, then the Border area around Krynn's been blocked. A body'd do well to stay away." And yes, Bruce Cordell does overdo the cant in that book.
Well, time might be different in the Ethereal. Ravenloft is an Ethereal demiplane and time acts weird there. And if publication date and this note on p16 is all that ties it down chronologically, no problem.
I have this one, so I can read it more carefully to see if it matters.
So while I completely understand leaning toward Spelljammer sources for a Spelljammer timeline, it may be worth noting that On Hallowed Ground and The Planewalker's Handbook aren't aberrations in this regard. Just more to ignore, I guess.
Not leaning, SJ products automatically trump other products if there is a conflict re: the SJ timeline. I knew conflicts would occur, and this made the most sense to me. Especially since the Krynn time sync is so specific in the first SJ novel, which is set on Krynn. DL really is the outlier here, they leaped way forward and messed with time anyway, starting in the Legends series. And then DL's sphere disappears.
So, for me, the SJ timeline actually makes by far the most sense even if I didn't have the 'SJ trump' policy. For a PS timeline, I think PS trumps would make more sense.
You left some page numbers in The Planewalker's Handbook with question marks, so I'll fill them in here.
'recently' is such a vague term... it helps a lot.