Activiti and external forms

Has anyone worked on fetching external forms in Activiti and vaadin.

Currently am developing my GUI using Vaadin and the BPM is Activiti. I have a user task in the Activiti Flow (process). Once the process comes to user task, a form (a java class, which is actually written in Vaadin eg. should be invoked, as described below. I went through User guide, but since am both new to activiti as well as java, can some one help me with an example? It will be great if anyone can help me on it.

A manager updates a bug report and assigns priority and target version.

can get any solution?

Please can you tell me the exact requirements both activiti and vaadin?