The full story is described in Srimad Bhagvata MahaPurana, Skandha 6, Adhyaya 9 to 12.
SB 6.9: Appearance of the Demon Vṛtrāsura
SB 6.10: The Battle Between the Demigods and Vṛtrāsura
SB 6.11: The Transcendental Qualities of Vṛtrāsura
SB 6.12: Vṛtrāsura’s Glorious Death
SB 6.9.18 — That very fearful demon, who was actually the son of Tvaṣṭā, covered all the planetary systems by dint of austerity. Therefore he was named Vṛtra, or one who covers everything.
SB 6.9.19 — The demigods, headed by Indra, charged the demon with their soldiers, striking him with their own transcendental bows and arrows and other weapons but Vṛtrāsura swallowed all their weapons.
SB 6.9.20 — Struck with wonder and disappointment upon seeing the strength of the demon, the demigods lost their own strength. Therefore they all met together to try to please the Supersoul, the Supreme Personality of Godhead, Nārāyaṇa, by worshiping Him.
Vishnu guided Indra and other Devas to go to Rishi Dadhichi which will award you his body. By making Astra (wapon) called thunderbolt (Vajra) from his body by Vishwakarma, Indra can kill Vritra.
SB 6.9.54 — When the Aśvinī-kumāras beg for Dadhyañca’s body on your behalf, he will surely give it because of affection. Do not doubt this, for Dadhyañca is very experienced in religious understanding. When Dadhyañca awards you his body, Viśvakarmā will prepare a thunderbolt from his bones. This thunderbolt will certainly kill Vṛtrāsura because it will be invested with My power.
SB 6.10.13-14 — Thereafter, King Indra very firmly took up the thunderbolt manufactured by Viśvakarmā from the bones of Dadhīci. Charged with the exalted power of Dadhīci Muni and enlightened by the power of the Supreme Personality of Godhead, Indra rode on the back of his carrier, Airāvata, surrounded by all the demigods, while all the great sages offered him praise. Thus he shone very beautifully, pleasing the three worlds as he rode off to kill Vṛtrāsura.
Vritrasura also accepted this:
SB 6.11.20 — O Indra, King of heaven, the thunderbolt you carry to kill me has been empowered by the prowess of Lord Viṣṇu and the strength of Dadhīci’s austerities. Since you have come here to kill me in accordance with Lord Viṣṇu’s order, there is no doubt that I shall be killed by the release of your thunderbolt. Lord Viṣṇu has sided with you. Therefore your victory, opulence and all good qualities are assured.
So, Vishnu helped Indra and for the way of killing Vritra. He guided him how to make Vajra (thunderbolt) that can kill Vritra and he also powered that Astra (weapon) so-that Indra could won Vritra. (It can also be said Indra got Vajra with the help of Vishnu).
