OK, cool. So when you were processing the data, it looks like a separate feature layer was created for each vegetation zone. What you would want to do instead is keep all of the vegetation data in the same feature layer, and have an attribute that describes the vegetation zones. You could then use this attribute to drive the symbology with the Type (Unique Symbols) drawing style. So its the difference between having many layers using the location (single symbol) drawing style drawing style to represent the vegetation vs having one layer using the types (unique symbols) style to represent the different areas.
We are looking to add support the location (single symbol) style in the next release of the interactive legend, but one of the reasons we didn't include it in the first release is that when a layer is symbolized in this way, the Layer List tool can perform the function of toggling the visibility of each layer on and off.
Here are some samples that will hopefully assist in your project.
I downloaded the dataset and then added a text field for the veg description, the resulting shapefile can be downloaded here.
a sample map that I made with is can be viewed here, and the resulting interactive legend app is here.
Let me know if this will work for you and if you have any other questions.
Cheers,
Ian