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);
}