To simplify things and help our users to be more productive, we have archived the current forum and focus our efforts on helping developers on Stack Overflow. You can post new questions on Stack Overflow or join our Discord channel.
Vaadin Framework 7.6.6 as well as the upcoming 7.7.x series are based on GWT 2.7.
We are unfortunately not able to directly include GWT 2.8 since Vaadin Framework 7.x preserves compatibility with Java 6 whereas GWT 2.8 requires Java 1.8.
Thanks to the Mavenized build that will be part of Framework 7.7, it should be easier to use your own GWT version together with the framework, and I would also expect any discovered incompatibilities with GWT 2.8 to be fixed swiftly as long as the fix doesn't break compatibility with the bundled version.