Focus on your vibrato depth. Depth is one of the two dimensions to vocal vibrato performance. Simplified, it is how big a change you are making with your voice. The challenge is to have enough depth to be properly heard so a note does not come across like one continuous flat sound but to not be so deep as to sound ridiculous (ah AH ah AH ah AH).
Adjust the speed of your vibrato voice. The second issue pertains to how fast you express the vibrato. If you vibrate your voice too quickly, it makes you sound nervous and jittery, rather like Alvin and the Chipmunks. The extreme opposite is someone who vibrates too slowly so that the listener's ears quit hearing the sound as a vibration.
Think of a vibrato as repeatedly changing your voice from loud to soft. Avoid lapsing into changing the frequency of your note, which results in the song sounding either sharp or flat.
Consider taking professional music lessons to improve the state of your singing. A well-practiced professional ear may be all you need to help you perfect both the depth and speed of your vibrato.