1.Keep your thumb over the neck and use your whole arm to do the vibrato not just the finger.
2. think of each pulse of the vibrato as a note of itself, which means keep it rhythmically even with the rest of the notes
3. for vibrato it helps if you don't press the string down with the actual tip of the finger but rather with the side of the tip so that you can move the string with greater control.
Hope this helps.
Mike.