More on Vibrato
1.Try playing without your thumb touching the neck of cello. Allow your released thumb to go wherever it wants to. If your thumb is beside you fingers, you may find greater freedom than if it opposite your second finger.
2. Place each finger with your entire arm, allowing your forearm to rotate slightly as you go from one finger to the next.
3. Release your fingers, allowing them to be close to each other rather than spread apart. Your hand, arm, etc. are freer with your fingers this way. Open the distance between fingers to reach for the next note but keep it closed as much as possible while vibrating on each note.
4. Pull your vibrato toward the flat side of the note. Firstly, because human ear accepts the top of the vibrato's oscillation as the pitch. Secondly, when you pull to the flat side of a note, your body's natural elasticity automatically takes it back to the note. Fore every pulled stroke you get an extra reactive motion with no additional effort. Two for the price of one!
So, the vibrato only goes from the pitch to the flat side and back, etc. It is the upper edge of your finger that establishes the length of string that vibrates and this is what determines pitch. This knowledge can help you establish a beautiful, tension-free vibrato.
A vibrato that is pushed upward doesn't return by itself, it must be actively returned. It requires more work,it feels tighter and tends to make you play sharp.
5. You might also experiment placing your fingers on the wood of the fingerboard between the strings and on the low side of the string and play by touching the low side of the string rather than the top of the string and pressing it down against the fingerboard.
This approach can provide the ultimate in freedom of movement. It is not necessary for the string to touch the fingerboard to produce good pitch and a beautiful vibrato. The only time the string must touch the wood, is when you play pizzicato.