Refer to page 511 (Figure 14.22) & page 514 (Figure 14.25) in UML superstructure specification 2.2.
(For convenient, the 2 figures can be saw in attachments)
The 2 examples shows that interactions can be put into a class in composite structure diagrams.
VP-UML didn’t provide a way to add a compartment to a class which contains interactions.
Although we can add sub-diagrams for a class, this feature is still not visualizable in a diagram.
It’s really important when we want to see what interactions are refered by a given class quickly, especially when we are reading a hard-copy.
Perhaps VP-UML can add a functionality for adding compartments to a class in composite structure diagrams, and then allows users to drag interaction diagrams(sequence, communication, and timinig diagrams) to there.