Version 1.15.1 for gstreamer and its plugin components has been released. The current version in portage is 1.14.x.
1.15.1 release is the first development series release, not stable. gstreamer follows the gnome release versioning in terms of stability - odd minor version number is development cycle and even minor version is stable cycle. I am unlikely to add those development versions before they become a stable 1.16.x series. Once a stable 1.16.x is released (I heard they hope later this month), you can expect a prompt upgrade to that.
OK. Is it the same for the plugin components? For example, to be more specific, media-plugins/gst-plugins-vaapi-1.15.1 includes many fixes.
Yes, it's the same for everything gst*. If there's some specific bug to fix in vaapi plugin, we could talk about that and backport the necessary fixes to 1.14.x though
*** This bug has been marked as a duplicate of bug 690468 ***