To be fair, Eskin's report seemed to say that Reid discussed making the change yesterday and would make a decision today. So either he had a change of heart or wasn't ever considering it in the first place.
The argument for keeping Vick in this particular week is really the same as it would be for starting Foles. The Saints are the lowest ranked defense in the NFL and just 28th against the pass. Just like it might be somewhat of a soft spot for a rookie to get his first start, it could also serve as an opportunity for Vick and the Eagles passing attack to get a rhythm.