When you expand a tree node the screen reader does not report a change of state. It should speak "Expanded". Conversely, when you collapse a tree node it should speak "Collapsed".

Investigate using ARIA to mark up expand/collapse state in regions. The number of subitems in the newly expanded tree should also be reported and spoken: "seven items" so the user understands how big a tree they are dealing with.

The next generation Vaadin tree grid component is more suitable for this task.