AMy friend is hosting a baby shower high tea for a mutual friend and has ordered a cake (you know baby bump bla bla) from me. The food will be cucumber sandwich with salmon, scones, ginger snaps etc. She was very indecisive about the cake flavor though. Anyway, in the end she decided to go with cappuccino cake, and "milky buttercream or so". Need some input if a sort of plain vanilla buttercream would be fine to complement the cake flavor or do you have any other ideas?

I think increasing you flavoring to 1 TABLEspoon of a vanilla mixture (to a recipe of b'cream using 2 pounds of powdered sugar)  would be just perfect. 

Try this mixture:  1 part vanilla; 1/2 part butter flavor; 1/4 part almond.   A 'part' is whatever measure  you use: it can be a teaspoon or a tablespoon or a cup or a gallon :)   Just keep the 1; 1/2;1/4 ratio. 

Some people don't like almond.  If you prefer you can use lemon extract OR coffee extract instead.   

If you make a Swiss Meringue Buttercream or Italian Meringue Buttercream - they will be more 'milky' (read 'dairy') than an American Buttercream.  Lots of vanilla flavouring will go nice with the cappucino, but so would a touch of chocolate - theory being that capuccinos have a dusty of cocoa ontop the froth .