Drainage fix: Flat concrete slab up over my vinyl siding by 2 inches. Is this acceptable?
From your question the part that jumps out at me is that you say it is flat.  Also that it is above your vinyl siding.  

I recommend first that you examine where it comes into contact with the vinyl and add some clear silicone if there are any gaps.  

Secondly, since you say it is flat, I would recommend you go out there with a hose and see how the water flows across it.  

You didn't mention if this is catching water from elsewhere or maybe from a roof above, but with the hose you should be able to simulate a rainy condition and then be able to make a determination about whether the fix is going to work for you.  

Good luck.