Why did Lord Vishnu need Rishi Dadhichi’s bones to kill Vritrasura?
When Vritrasura (वृत्रासुर ) was not defeated by the existing weapons, so devata went to Lord Vishnu for suggestion as what can kill him.
अथो ईश जहि त्वाष्ट्रं ग्रसन्तं भुवनत्रयम् ।
ग्रस्तानि येन न: कृष्ण तेजांस्यस्त्रायुधानि च । – Bhagavata Purana 6.9.44
O Lord Krishna, please annihilate this demon Vritrasura, Tvasta’s son, who has already swallowed all our weapons, and now he destroying the three lokas, you please kill him.
मघवन् यात भद्रं वो दध्यञ्चमृषिसत्तमम् ।
विद्याव्रततप:सारं गात्रं याचत मा चिरम् ॥ – Bhagavata Purana 6.9.51
ततस्तैरायुधश्रेष्ठो विश्वकर्मविनिर्मित: ।
येन वृत्रशिरो हर्ता मत्तेजउपबृंहित: ॥ – Bhagavata Purana 6.9.54
Lord Vishnu said – please go to Rishi Dadhichi (दधीचि) and ask him his body, which is hardened by his fasting, prayer and tapasya. Vishwakarma (विश्वकर्मा) will made a weapon out of his body parts, using that weapon Indra, you can cut the head of that demon.