I would check Weird Tales from, say, 1928-1933 (age 12 to 17). Jack or someone in the household had a subscription to WT when he was living at the ranch. Somewhere he mentioned that he would walk down the lane to the mailbox with anticipation when a new issue was expected. I have also seen a reference to Amazing Stories, so it is worth checking also.

If by some miracle Vance shows up at Marcon over Memorial Day Weekend, one question I would like to ask is whether he submitted the original Dying Earth stories to Weird Tales. I can see why the editor of Startling/Thrilling Wonder considered them unsuitable for his magazines, but I can't believe that Weird Tales wouldn't snap them up!