So I completed the story before reset on the first day and was able to buy another Grawl Firestone from the Heart vendor for the daily after reset. Later that day I started replaying the episode to work on the instanced story achievements. I found that today I was unable to buy the firestone, but I had a hunch. I went back and replayed up to the point of getting the firestone the first time and not only did the grawl give me another free firestone it reappeared as purchasable on the vendor. In essence if the bug you are experiencing is the same as mine then its caused by replaying the episode resetting the flag that allows you to buy the firestone from the vendor.
This Works..Just do the story to the firestone part and bug fixed