Is the auction quest the only opportunity to acquire the viper gear diagrams? Amazingly, I didn't have enough cash to acquire it all(heart of stone is quite cash intensive), am I looking at a reload of an earlier save? I really hate when games cut you off without warning.

I haven't found the seller after the fact. Perhaps they hang out somewhere in Oxenfurt, but the wiki says otherwise. However, you can find the viper silver sword during the final quest against O'Dimm. Its to the left of the map in a stone up a set of broken stairs.

Ended up reloading, and heading to the auction house fully minted (instead of firing 5000G at a particular crafting quest) AND BOUGHT EVERYTHING ON OFFER!