Will the result be similar to using regular BC?
it will be exactly the same.... or at least it was when i did it! If you only using a few drops of extract it shouldnt alter the consistency of the BC
What do you mean by vanilla icing? What is your recipe?
For my FBCTs I have always used my regular all crisco buttercream that I use on my cakes. I've never seen the point of doing the recipe for an all butter buttercream that I have seen associated to them.
If you are unsure if your recipe would work, then I would pipe some on to a piece of wax paper and stick it into the freezer. Once it is frozen see if the icing will release from the wax paper (or what ever you choose to use). If it releases with out a problem then the icing will be fine for the transfers.