Here’s a quick tip to get your vibrato working. Stand in front of a mirror; press on your chest with both hands, then, raise your chest higher than normal. Take a breath in and then exhale, but don’t drop your chest. Sing one note and hold it as long as possible with your chest raised. Press on your chest halfway through the note (press kind of hard and raise your chest to meet the pressure). Relax the back of your neck and keep your jaw open as you’re singing “ahhh.” Imagine the air spinning around in your mouth while keeping your chin tucked down a bit and your chest raised. Keep in mind, overuse of vibrato is not a good thing in contemporary singing (pop, rock, and R&B). At the same time, no vibrato is also not a good thing. So, try ending phrases with straight tone, then into a little bit of vibrato. The bottom line is to do what’s best for you.