I created a new Liferay Vaadin portlet with Liferay IDE. In this portlet, I added a custom widget I’d like to compile. But I can’t because my project is not recognized as a Vaadin project.
https://vaadin.com/directory#addon/vaadin-control-panel-for-liferay should help you. Basically in a portal the widgetset (and the Vaadin jar) is in the portal and not in the project so you need to add the add-ons to the portal and recompile the portal widgetset (using the control panel add-on).