I'm in the opinion that even if they are not in your plans, and you don't believe long term in them as prospects - you still reward someone who's doing well with a call up when you don't have a block. Vaughn is not a prospect. He's not blocked. Been spoon fed pretty much his whole career. He hasn't worked out. Vaughn is not a block.
When you show your actual prospects that you will reward outstanding performance by calling up the players, it builds a good morale around the place instead of creating a jailed mindset; "No matter what I do, I'm never getting out of here so why should I try hard"
Lastly, I honestly believe that Getz is waiting to trade Robert, then will promote Elko. What does one have to do with the other? To me, nothing. To Getz, he probably sees it as, it frees up a roster spot, and I'm not creating an Elko vs Vaughn thing. The problem is, the fans have already done that by not calling him up. Elko could DH. Or Vaughn can. Heck, send Vaughn back to the OF for all I care if you're adamant that he has to play for the Chicago White Sox.