Well, first think about what vibrato is. Simply, it's the moving back and forth of the vocal chords as air passes through them. Also, everyone has their own type of vibrato, whether it be subtle, obvious, slow, fast, etc. The trick is find YOUR OWN natural vibrato. DON'T try to imitate anyone else's. Your vibrato comes through at its truest when all of your muscles are relaxed. Make sure you're not gripping your ribcage, or tensing your tongue. The throat should be nice and open, as if you are about to yawn. Breathe deeply, and fill in your back with air. Make sure that the note is resonating, meaning it's moving around somewhere in your head or chest, and that you're not just sitting on the note. The presence of a natural vibrato suggests a healthy and proper technique, so you should definitely work on it. Just let go, relax, and let your true voice shine through :)
Opera is when a guy gets stabbed in the back and, instead of bleeding, he sings.