The great thing about kölsch in homebrew is that you have some latitude in how you want to brew it. After some iterations, I found that 95% pilsner malt and 5% light Munich malt hit the flavor spots I crave. Your tastes may vary...an experiment with Vienna would probably be neat! Personally, I don't want too much character on the malt side, because kölsch should be a "delicate" style (in my opinion). Too much Vienna might throw things out of balance for me. But, likely worth a try if you are curious.