A cup of coffee for Vargas wouldn't have been a bad idea. He could function as a pinch hitter and be an upgrade over Herrmann, but in the end, I'm not sure this is really going to have much of an impact either way.
I suspect Colabello is going to get optioned on Saturday for Arcia or sooner for Willingham, and Herrmann will get returned to the minors as soon as the other is ready.
I'm not a fan of calling guys up from AA for short-term bench assignments. If he's ready to skip a level, he's good enough to start at the DH position. I don't think that's the case. With Willingham and Arcia a week away, it makes sense to use Hermann as filler. He'll be a Red Wing again before the month is out.