Find a Guo-Lai Vault Key and open an Ancient Guo-Lai Cache.
The caches spawn randomly in the halls (and the achievement is misnamed to say "Vault Key"), but I can confirm you can use this at any rep (friendly right now), though the more unlocked areas, the easier it is to find a cache. Once used, the cache vanishes. You need one Ancient Guo-Lai Cache Key
For me, I got 1 Spirit of Harmony, 1 Relic of Guo-Lai, and 21gold. The relic gave me 180 reputation with the Golden Lotus