Elvis used vibrato in every song. He couldn’t do those slow songs with excellent vibrato without vocalizing practice.
They Remind Me Too Much of Youl by Elvis
Vibrato takes 60% more breath to sing. Audiences are soothed by vibrato. Imagine you are soothing the audience while you sing vibrato.
Simple Vibrato
Vibrato should be practiced while using a metronome (on your phone). Your vibrato cycles count must be in sync with the beats of the metronome (6 to 7 pulses a second [Htz]). Slower than 5 Htz is a wobble. Faster than 8 Htz is a tremelo. You can practice those too.
Everyone can do it. You must start small and practice a lot. Take a song you like with a lot of vibrato. Sing a vibrato part very very low in volume, just above a whisper. Notice how Elvis lowered the volume of his voice when he sang a vibrato part. After you are consistently singing the vibrato pulses, practice it at low volume ALL THE TIME -- when driving, when at Walmart, when at the grocery store, etc....