IE 7 support was dropped in Vaadin 7, but are there some possibilities to disable error message and to let users to use an application on their own risk?
There is no way Vaadin will work in a vanilla IE7 because the bootstrap mechanism uses JSON.parse which was introduced in IE8. If you find a workaround to that as well as all the layout issues you will get from lacking support for the CSS property box-sizing: border-box which will be used extensively, you could have a look at the method WebBrowser.isTooOldToFunctionProperly() and the class UnsupportedBrowserHandler.
When I use Vaadin 7 on IE9 my simple LoginForm don’t have any styles applied to it.