There are two ways to do this: 1. In liferay, you can redefine custom jsp pages using hooks , on this redefined jsp page you can specify your vaadin portlet using jsp tags, maybe you should redefine authenticateByScreenName / authenticateByEmailAddress (... ) in UserLocalService (you can also do this with hooks)
2. use sets up a custom action in liferay , this can be done with hooks, I think this is a better idea
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