Want to participate in the community, and help others like yourself use Vaadin? Great – you'll find some good starting points here and the best part is you'll learn so much at the same time.
The easiest – and at the same time one of the most important – way of contributing is reporting issues you have with Vaadin. That is, annoying bugs, features that you would like to see or just general improvement suggestions.
Create and publish add-ons
Is there a feature missing from the core? Sometimes it is faster to fix it yourself than wait for the core development team to do it. The great add-on and extension architectures in Vaadin allow you to easily include all sorts of awesome stuff on top of the Vaadin core.
Once you've written an add-on, why not share it with the rest of the community, so everyone can benefit?