This may not be a bug, but it seems if you do not have a rank 3 available to purchase(not that your missing currency, but that you have one available that a different character acquired of rank 3) the vendor does not become available. All you need to do is have a rank 3 essence available on a character that the character your on does not have.
It’s either an oversight of blizzard coding the vendor, or a bug where the vendor doesn’t become available unless you have a rank 3 you can purchase that you don’t already have.