The best way to run videos on a different app is by first trying to resolve the package, so something like this:
public void playVideo(String key){
    Intent intent = new Intent(Intent.ACTION_VIEW, Uri.parse("vnd.youtube:" + key));
    // Check if the youtube app exists on the device
    if (intent.resolveActivity(getPackageManager()) == null) {
        // If the youtube app doesn't exist, then use the browser
        intent = new Intent(Intent.ACTION_VIEW,
                Uri.parse("http://www.youtube.com/watch?v=" + key));
    }
    startActivity(intent);
}