I installed the Eclipse plugin a while back from the plugin site (http://vaadin.com/eclipse) as per http://vaadin.com/eclipse. My current plugin version is listed as 2.0.1.2011.05170102. I noticed, however, that the latest plugin version is listed as 6.8.3.201209051212.
The package with the version number 6.8.3 is simply versioned based on the bundled Vaadin version - it is based on exactly the same source code as the one you see on the main update site and the only difference should be the bundled libraries.
The version on the update site is the stable version of the plug-in and is updated occasionally, when there is a reason for such upgrades. The always current (possibly unstable, although usually in good condition) development version can be obtained from the experimental update site (mentioned above). It has some new features under development, possibly some bug fixes that haven’t been released yet, partial support for the next major version of Vaadin etc.