I am thoroughly enjoying this game, so I built more than one vault to keep me occupied while one is in downtime. I find that sometimes I obtain items or dwellers in one vault that would be helpful for the other. Seeing as how these vaults are all in the same wasteland, as per the lore, I would like to see a feature implemented that would allow aplayer to trade items, dwellers, etc between that person's own vaults.
Also, in an unrelated note, allow us to sort the dweller list in descending AND ascending order. I often use the dweller list to find my least happy dwellets, and getting close to 100 or more dwellers makes it quite a pain to continually scroll all the way down. I felt this suggestion didn't warrant its own topic.