Vendors don`t sell Dragon no matter how high your level in my experience.
My current character is at level 31 and the vendors don`t even sell Ebony except for arrows and ingots.
I do regular tours to easy-to-access Boss chests that I know will respawn.
No Daedric has ever been found and extremely rarely a Dragon piece will show up.
It`s an entirely random long shot but I`m guesstimating @ once in every 40 or 50 chances.
Top end loot is usually Glass or Ebony but I`ve discovered only an unenchanted Dragonplate cuirass, Dragonscale Armor of Eminent Conjuration , Dragonplate Armor of Invigoration, Dragonplate Gauntlets of Extreme Wielding , and Dragonscale Gauntlets of Peerless Alchemy out of well over 200 times I`ve popped such chests.
These odds may be improved a little if you have the Treasure Hunter perk from Lockpicking
A higher level character of mine gets slightly better results including Daedric but I don`t know what the level is that the chest list/odds change