For many players, the optimal Great Vault pick is a 
Nebulous Voidcore to use on a boss of your choice and receive an item from that boss’s loot table. On Heroic difficulty, a bonus roll will yield a Myth track item.
By utilizing the Crest Upgrade system, you can upgrade that Myth track item from 1/6 to 2/6 without spending any 
Myth Mistcrests.
How Crest Discounts Work
If you upgrade a piece of gear to a certain item level, upgrading a new item in that same slot up to the same item level costs 0 crests.
Rings, trinkets, and dual-wield weapons require both slots to be upgraded to that item level before the discount is activated.
How to Save Crests When Upgrading Items
If you plan to use multiple Bonus Rolls on a specific boss, you can get an idea of which items you will get on Myth track and avoid spending any crests in those slots until you can upgrade them for free.
You can upgrade a Myth track item from 1/6 to 2/6 for free by utilizing crest discounts. To do that, you will need a Hero track item in the same slot. Having a Myth track item at 1/6 (318 ilvl) will let you upgrade the Hero track item to 5/6 (318 ilvl) without spending any 
Hero Mistcrests.
Then you can upgrade the Hero track item from 5/6 (318 ilvl) to 6/6 (321 ilvl) by spending 20 Hero Mistcrests. This will let you upgrade the Myth track item from 1/6 (318 ilvl) to 2/6 (321 ilvl) for free, saving you 20 Myth Mistcrests.
TL;DR
- Have a 1/6 Myth track item (318 ilvl) and a Hero track item of the same slot.
- Upgrade the Hero track item to 5/6 (318 ilvl) for free.
- Upgrade the Hero track item from 5/6 (318 ilvl) to 6/6 (321 ilvl) for 20 Hero Mistcrests.
- Upgrade the Myth track item from 1/6 (318 ilvl) to 2/6 (321 ilvl) for free.
