I'm not sure about vegan, but I would assume you could just substitute the liquid with champaign. I've never done a vegan cupcake, but I assume you can't use butter (correct me if I am wrong). What product can you use?
I've made a moscato cupcake with moscato buttercream before and it was good. For the BC, I just made my normal BC, but instead of putting heavy cream in it, I added in a couple tablespoons of moscato, just till it hit the right consistancy. The frosting had a nice moscato flavor, but wasn't overwhelming. Everyone loved it.
If you like, you can use the LorAnn champagne extract. Makes great nonalcoholic icing.
For vegan buttercream, most recipes use vegan margarine and icing sugar. Remember that these special margarines have more water so add liquid carefully.
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