For the current release of the Visual Vocal system there is not currently a way to view user response rates within the web portal. As a Project Owner, though, you can generate a spreadsheet of user responses for a particular V|V. Perusing this data will alert you to users who have and have not responded. You can then also use various visualization tools to understand response trends.
For information on generating the output response spreadsheet, refer to the article How Do I See Feedback That My Users Have Submitted?