The bell is indispensable musical as instrument in tantric Buddhist ritual, when performing rituals they held along with a thunderbolt in their hands. The sound of the bell is used to drive away evil spirits. It sends out a warning to evil spirits that they may not come to the place where ritual is being performed, this is usually consecrated ground.
The vajra represents compassion and the masculine side as well as the mind of Buddha, the bell represents wisdom and the female side as well as Buddha's body.So in order to achieve enlightenment they must be combined.
The use of the bell and the vajra differs with every ritual as well as the sadhana  chant. If it is necessary to evoke demons or to visualize them, the vajra is used to call them and the ringing of the bell to protect them from the demons. It can also be used as a sound offering, when evoking the deity Vajrasattva the vajra must be place on the chest of the meditating thus symbolizing that they are one inseparable. And then with the ringing of the bell this symbolizes attainment of wisdom and understanding the emptiness within. During this ritual the vajra is place in the right hand face down, the bell is placed in the left face up, the hands are crossed and place against the chest representing the union of male and female.