For the pale/vienna malts you could substitute Munich extract, which I believe would be a mix of pale and munich extracts. I've never heard of Vienna extract, but that doesn't mean much.
You can steep the caramel 20 grains.
As for the corn, I think it has to be mashed, but if it were me, I'd just leave it out. What corn does is lighten a beer, add fermentables without much flavor. You could, I suppose, sub in a little bit of corn sugar for the flaked corn.
What kind of yeast are you using? That's very low IBUs, by the way, even for a lighter beer.