After Liferay SDK plugin installation all my Vaadin projects created via Vaadin plugin were marked erroneous. Problems view reports the message 'The SDK name of this PluginSDK project is not set'. Obviously Liferay plugin means my Vaadin projects are Liferay Plugin Projects. Note my Vaadin projects are not Vaadin portlet projects, just old plain Vaadin projects created via Vaadin plugin.
Due to this bug Liferay Plugin and Vaadin plugin cannot co-exits in one Eclipse instance.
See also discussion