I will argue against your point, since Vital Strike says that it can be used as part of an Attack action, which is exactly the action you use with Corrupting Touch, despite them both being described as "special."
As quoted, Corrupting Touch describes it as a standard action, not using the wording of it being a generic natural attack. This wording is also the sort of thing that excludes a lot of actions from working with Vital Strike.
One could make the argument that it is an incorporeal touch attack that follows normal natural attack rules, but then there shouldn't be need for the text on "as a standard action", implying that it is a special action type.
I will agree that it isn't the most well telegraphed fight either. It comes out of nowhere with an enemy type that tends to be much stronger without preparation. While it certainly creates a rough fight and a strong climax boss encounter, it does seem like bad design mechanically.
...also, I am pretty sure we ruled the bite attack was corporeal, as an aside. I think that was clear from the statblock, but now I am less sure.