You can ask all the questions on your mind on our forums (http://vaadin.com/forum/). You don't need to attend JavaOne to get the hard questions answered.
Yes, Vaadin is a server-side framework. Server-side is not for everyone, but there surely is a place for Vaadin. For example issues with trusting the code that's executed on the client are greatly relieved this way.
You could think of Vaadin as the Swing / SWT for web applications. Sitting in between JavaFX (plugin required) and Java Server Faces (html required). Vaadin is great for desktop-like applications in the webbrowser, but doesn't really fit the RESTful type of applications.
If you have questions, just visit our forums at http://vaadin.com/forum and ask away! There are plenty of experts waiting for challenging questions ;)