Not sure what you mean - again, I manually configured them - one at a time directly on their web interface and not via Awesome Miner.
This does not stop AM from actually changing the setting assuming there is a glitch in AW program itself (not the firmware).
Hopefully it's not some BS from NH, but with the miners configured to maintain temps at 76 and plenty of fan power - I don't understand how NH could cause their temps to go high enough to cause them problems...
If you don't have a second pool setting in place, and assuming for whatever reason the connection to NH is lost due to whatever reason, be it your side or theirs, the miners will run pretty hot and might restart themselves, that is a known problem in the default firmware I am not sure if Vnish works differently.
... and doubly so when AM logs don't show any temp spikes.
Assuming the above scenario is true, AM won't have access to the miner, it will show as offline as long as no pool is showing "alive" on the miner.
All the above is rather speculations with a pretty low probability of being the real cause, the most likely cause of all this mess is NH, we know that NH causes some chips to disappear or show xxxx, all those chips are connected to each other, so if a few fail the rest will too, that might also have to do with why the setting changed, maybe Vnish is set up to auto-tune when some chips fail and thus all of the previous setting you saved were overwritten.
As I mentioned before, there is really no technical explanation on how or why does NH cause the issue, the only semi-proper explanation we have so far is the non-stop changing for difficulty, I had the same problem using the default firmware, switched to a proper pool, some of chips never showed up again, some did and worked without an issue ever since.
Long story short: start by changing the pool.
Good luck.