瓦斯卡尔恩 in the Central Encampment of the Emerald Dream is the vendor for the Dreaming Crest trade-up./way #2200 49.80 62.94 Dreaming Crest Exchange
You won't see the trade-up options unless you reach certain item-level thresholds. (Devs call them High Watermarks)
15x Dreaming Crests are worth one item upgrade.
Even in the latest PTR Release Candidates, 483 crafted gear in every slot only unlocked the first trade-up. Maybe it was not implemented or time-gated... However, this "Developer Insight: Universal Upgrade System Improvements for Dragonflight Season 3" Article confirmed that they intend to use the 6:1 ratio for all trade-ups, and earning 90x lower rank Crests and trading them for 15x higher rank Crests will count towards the weekly cap of both Crest types.

Unlock Achievement	Requires	Vendor Addition	Cost
幼龙之梦	Reach a High Watermark of 450 in Every Slot During Dragonflight Season 3 (Outgrow Whelpling's Dreaming Crests)	1x 一袋闪耀的幼龙的酣梦纹章	90x 雏龙的酣梦纹章
魔龙之梦	Reach a High Watermark of 463 in Every Slot During Dragonflight Season 3 (Outgrow Drake's Dreaming Crests)	1x 一把闪光的魔龙的酣梦纹章	90 x 幼龙的酣梦纹章
守护巨龙之梦	Reach a High Watermark of 476 in Every Slot During Dragonflight Season 3 (Outgrow Wyrm's Dreaming Crests)	1x 一捧绚灿如花的守护巨龙的酣梦纹章	90 x 魔龙的酣梦纹章

I cannot trade any 幼龙的酣梦纹章 (got 405) for 一把闪光的魔龙的酣梦纹章. I am GS 481,5 and have all 3 achievments concerning the Dreaming Crests. I just get the message "Item is locked".

You cannot purchase this item and will say locked if you reach your weekly cap.

小知识

屏幕截图

视频

一把闪光的魔龙的酣梦纹章

物品等级：1
拾取后绑定

使用: 收集15个魔龙的酣梦纹章。
需要等级 70
你现在还不能获得15枚魔龙的酣梦纹章。

最大叠加: 50

[Vendor Locations]

[This item can be purchased in] 瓦德拉肯（3）和翡翠梦境 .