AT the moment I have got the Breastplate of the Dragonheart AND the Breastplate of the Dragon AND the Dragonheart Gem... How did I do this? Well immediately after I had received the Breastplate of the Dragonheart from Grischa, I talked to him again, aiming to buy a better axe... At the top of his itemlist were both the Breastplate of the Dragon and the Dragonheart Gem. It makes you wonder why he had you searching for the two parts, as he clearly had the Breastplate of the Dragonheart all along... I wonder if the same is possible with the Larian Armor Quest.
P.S. I tried to get Grischa to make another Breastplate of the Dragonheart, but there was no such option.