A demo would be nice. Here it seems like parent nodes are selectable; at least, it is weird that the type is the same for tree parent nodes and tree leaves. In fact, if I have FreeUniversityOfBerlin->DeptOfMathAndComputerScience->DeptOfComputerScience then the root is not really a department and the middle tier is not something I would want to be able to setValue().
This is somewhat limited to how Vaadin's DataProviders work, but it can be done by clever design of POJOs, see the question by Tom Eugelink here and my answer to it