Board Thread:Questions and Answers Forum/@comment-76.4.84.215-20150721164145/@comment-24057866-20150721194205

For Vault of Glass, there's really no reason to run it on Normal if you can run it on Hard. If you run it on Hard mode first, you get loot for both Normal and Hard, and then you are locked out of loot for that week for both difficulties. So run it on Hard once and then you're done.

Crota's End is different. As you can see from the List of Loot in the Crota's End article, special and heavy weapons only drop on Normal mode and primary weapons only drop on Hard. Hard mode still pays out loot twice (once for both difficulties) like Vault of Glass, but unlike Vault of Glass, it won't give you anything from the Normal-exclusive table; you'll just get Radiant materials and armor pieces.

So if you're trying to get a special weapon like Black Hammer, you must do Normal mode before doing Hard. If you do Hard first, you won't get anything on Normal because you'll already be locked out.