Good question and multiple answers.
[b]
- Contributing back to official Vaadin projects (Vaadin Framework, Vaadin TestBench, …)
[/b]
Please send patches attached to tickets at dev.vaadin.com. In many cases, the patch is used as a reference and core development team rewrites the feature. For more sizable contributions that are integrated directly integrated you have to sign a contributors agreement moving the rights to the code to Vaadin Ltd to keep the IPR:s for Vaadin projects clear and safe everyone involved. If you become a regular contributor and several of your patches get integrated, you will be considered for a commit access to the project directly.
2) Contributing back to some other project in dev.vaadin.com/svn (incubator or contrib)
Contact the author of the project. If he accepts you as a committer, you can contact me to get commit access.
3) Contributing back to project outside *.vaadin.com (for example github or code.google.com)
Contact the author. He can also arrange you a commit access.
4) Creating a new project in dev.vaadin.com/svn (incubator or contrib)
As there are already tons of projects, it is better in most cases to use code.google.com, githug or something else. If you really really want to host your project at svn.vaadin.com, contact me.
5) Creating a new project outside *.vaadin.com (for example github or code.google.com)
Feel free to do so 
Whenever you are creating a new project, please also consider the guidelines for name use. (cut-n-paste from the
http://vaadin.com/terms-of-service
):
