A note of caution with EclipseLink regarding issues with Java 8 Stream API (since I believe Vaadin 8 relies on streams under the hood). Make sure to use the latest version of EclipseLink and verify that you are not being affected by this bug.
Apparently it is fixed in 2.7.0 but hasn’t been formally released yet (nightly builds are available).
https://bugs.eclipse.org/bugs/show_bug.cgi?id=467470
Or maybe consider another ORM provider such as DataNucleus.