The vCD API has a Vapp.GetChildrenVapps method, that returns a list of vApps that are nested within the current vApp. Presumably, each child vApp can have child vApps of its own, so that one can create a whole hierarchy of vApps within a single vApp.
However, I couldn't find a way to do this from the vCD UI (I'm using vCD 5.1). Is it possible to have a vApp within a vApp?