For any comment, suggestion, doubt or problem report related to VNUML, you can send a message to the vnuml-users list (remember to change the "at" in the address by an "@"). You can subscribe to the list or just have a look at the archived messages.
There is also a vnuml-devel list to discuss development issues. Here is the archive.
If you want to contact directly VNUML development team, you can use the following address vnuml at dit.upm.es.