See this post for a nice example how one can display the level of confidence in the data presented for a mapping unit on a map. The SoilWeb provides the distance from the nearest map unit polygon boundary. It gives the user a little more information that can be used to judge a relative degree of confidence in the returned soils data; the closer you are to the boundary the less likely you are actually standing in displayed map unit. Simple but useful, especially for small screens.
This could be useful for vegetation maps too. Unlike the sharp boundaries drawn on the map, transitions between vegetation types are often gradual along an ecotone. Thus vegetation closer to the boundary will show more of a blend of characteristics of the two bordering vegetation types. The next challenge would be including information about the width of the ecotone, but that is going to be difficult as this may vary from location to location.