Hmmm....it seems that my work here may be done.
I only originally looked because a crash mentioned a feature I'd added (3 years ago?).
That was a failure by the plugin to pass on parameters in an agnostic/generic way.
Now fixed.
Then there was a problem with a recently changed API in Vix that was silently trapped by a try/except clause to do nothing - not even mention the problem.
Fixed (although, to be fair, it still wouldn't report failures).
Then it was found that the same API change had resulted in a third piece of code never being reached at all.
Also fixed.
From what I gather the settings for VPS now work as intended?