If you don't want a MDI for your web application, simply switch your application style to web style (see Vaadin Application style).

An example configuration:

<servlet>
   <servlet-name>WebServlet</servlet-name>
   <servlet-class>com.sibvisions.rad.ui.vaadin.server.VaadinServlet</servlet-class>
 
   <init-param>
      <description>Vaadin UI class to use</description>
      <param-name>UI</param-name>
      <param-value>com.sibvisions.rad.ui.vaadin.impl.VaadinUI</param-value>
   </init-param>
 
   <init-param>
      <description>Application widgetset</description>
      <param-name>widgetset</param-name>
      <param-value>com.sibvisions.rad.ui.vaadin.ext.ui.Widgetset</param-value>
   </init-param>
 
   <init-param>
      <param-name>main</param-name>
      <param-value>com.sibvisions.apps.projx.ProjX</param-value>
   </init-param>
 
   <init-param>
      <param-name>config</param-name>
      <param-value>/com/sibvisions/apps/vaadin/web/application.xml</param-value>
   </init-param>      
 
   <init-param>
      <param-name>Application.setup.classname</param-name>
      <param-value>com.sibvisions.apps.vaadin.web.WebApplicationSetup</param-value>
   </init-param>
</servlet>
 
<servlet-mapping>
   <servlet-name>WebServlet</servlet-name>
   <url-pattern>/web/*</url-pattern>
</servlet-mapping>
 
<servlet-mapping>
   <servlet-name>WebServlet</servlet-name>
   <url-pattern>/VAADIN/*</url-pattern>
</servlet-mapping>

The important thing is com.sibvisions.apps.vaadin.web.WebApplicationSetup because this class changes the default style of ProjX.

The configuration doesn't change your workscreens, but it changes everything else. It removes the menubar and toolbar. Your workscreens will be shown as pages instead of windows. The UI is clean and simple because a web application should be easy to use.