It may be mods, yeah.
- With mods enabled I got the vendor bug reliably.
- I thought it might have been one of the Dye mods since that was all I bought from the vendor, but even with those disabled the save error occurred.
- I purged all mods and tried with the game I'd been testing on and it wouldn't even load anymore (I think the code was 522)
- I used a very old save that I hadn't had many mods installed with and could buy from every vendor in the grove and save reliably.
So, don't get too angry at Larian, seems like it may be my addiction to mods that is the issue. Now, the rampaging Ox is probably NOT that.