yea my closest lhbs only has carafa spec II as well. The one where I used to get III is closed now. It's only for color anyway. Special won't much affect the taste since it's dehusked. You could even use midnight wheat which I've started to use more for mouthfeel and color.
Dark Munich is not real common. Not many people use it. I'm lucky that my LHBS carries Avangard Best Malz because I love all their grains. The dark gives a little more of that sweet malty caramel flavor along with the usual bready munich flavor.
You may want to dial back the IBU a point or two depending on how you want your vienna to taste. I don't like mine sweet at all so I probably increased hops for the dark munich. My latest vienna was closer to 30IBU. If you're using brewing software just shoot for midrange bittering for the style. I think the original recipe from @mattnaik was closer to 21IBU with the bittering charge at about 18 IBU. I now use german magnum for bittering to about 22-23 then another 5 or so IBU at 10 mins or so. Again, I tend towards a little higher bitterness within(usually) style guidelines.
Speaking of IBU, hops AA percent will vary by crop so use the numbers from your actual package and plug that into your software along with the changes to your grain bill.