That's a very interesting idea. I have a few nitpicks with some of your assumptions that boil down to "but if the Vulcan scanners weren't that sophisticated, they wouldn't know," but even then, there are quite a few oddities surrounding the flight of the Phoenix (like the fact that most of Cochrane's techs were killed a day earlier.) Something fishy was definitely going on, and remember, the Vulcans then didn't believe in time travel, so alien intervention would be a logical guess.
At the same time, the Vulcans did know about Earth & humans prior to FC. I'm pretty sure I wouldn't trust any group today with warp drive technology, so Ican imagine the Vulcans felt the same way.
Still, you make a good case for the Vulcans to say, as you put it, "WTF?"