Remember that a cellos vibrato naturally goes flatter than the original pitch, so if it sounds just barely flat to your ear than its probably correct. Additionally, more rosin on your bow will increase its friction against the strings which will allow for deeper and more controlled vibrato so long as you never vibrate sharper or towards the floor. Correct vibrato requires your elbow to stay stationary while your arm merely rotates to move your palm. You might also consider playing with the pads of your fingers, rather than your fingertips.