I read the GWT are not compliant with IE 7 - 8 is It true? If is true VAADIN will not work??
It should be a problem.
GWT supports IE6-IE8 (
http://code.google.com/webtoolkit/doc/latest/FAQ_GettingStarted.html#What_browsers_does_GWT_support?
) and so does Vaadin (
http://vaadin.com/features
)
Thank a lot for the quick answer!!! 
However, take a note that Google has stopped support for IE6 - thus, future versions of GWT may not be 100% compatible with it as well as Vaadin in some far future.