Try using a FrameLayout instead. I’m not sure why, but if I use a Linear or Relative in my code it won’t center, but FrameLayout does. Here is the XML that fit my video to the screen, preserving the ratio and centering it:
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:background="@drawable/bg">
<!-- Video player -->
<VideoView
android:id="@+id/surface_view"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:layout_gravity="center"/>
</FrameLayout>