Hi everybody,
finally I had some time to check into this issue.
I think, writing an e-Privacy Plugin for Joomla! that covers all the country specific demands is a big and tricky project and I think that the developer(s) have done a good job, although the use of the plugin has some impact on the use of your website.
After installing the plugin on my test system I can reproduce the problems which Frederic has encountered. When you use a form in a module position the message COM_VISFORMS_FORM_MISSING (which in this case was not translated by the - I think French - language files, than) shows up. If you display a form using a menu item, just nothing happens, when you press the submit button.
This problems only occur as long as the user has not agreed to the us of cookies in the e-privacy plugin.
The problem is caused by the way the plugin handles requests, as long as the user has not accepted the use of cookies regardless of whether a component uses cookies or not (Visforms does not use or need cookies). As far as I can see, there is no way to make changes to Visforms so that it could still work, if a user has not accepted the use of cookies. Login in to a website (frontend) with the Joomla! login does not work either and I think that every component with "Submit Buttons" will fail, too.
So if you want to use Visforms and the e-Privacy Plugin on the same website I can only see the option to change the plugin text in a way that it is clear to the user that they cannot use forms on the website and submit data, as long as they have not accepted the use of cookies. (Which is a bit sad, because the user will think, that this features actually does use cookies, which is a wrong conclusion).
Regards,
Aicha