Actually, Vritra was the one success story of Indra. If you go by ACKs (I'm unsure of the original puranic/vedic source), there are 2 stories about it - one in 'Indra & Sachi', and the other in 'Indra & Vritra'.
The former has the one that has been somewhat shown in this serial so far. Indra felt threatened by Trishiras and therefore murdered him w/ his Vajra. This brought about Vritra being sent to go to war w/ him, and Indra initially suffered defeats, and so signed a temporary treaty w/ Vritra, pledging an end to hostilities. Vritra sought from him a promise that Indra wouldn't try to kill him either during day or night, w/ any weapon, or anything wet or dry. Indra kept his word for a while, but one day, at the beach, when he & Vritra were there at twilight, he got his chance. There was a rich amount of foam, which is neither wet nor dry, it was neither day nor night. Vishnu entered the foam, and Indra hurled it at Vritra, slaying him. After that, Indra went into penance for breaking this vow, as well as killing Trisiras, and that's when the Nahusha story (of him becoming ruler of the devas) started.
The other story was pretty different, and probably the Vedic Indra, if I'm not mistaken. Here, on one occasion, Indra and his devas were so lost in apsara dances & entertainment that when Brihaspati came in, they overlooked him, and an angry Brihaspati abandoned the devas. Uptil then, the devas would win the devasura wars, but this incident was followed by their losing. So Indra approached Brahma for advise, and he suggested getting a Guru who would do his yagnas and help them regain their edge over the asuras.
Now, here is where the story gets interesting. Brahma suggested Trisiras' name to Indra, and Indra raised his reservations, given that Trisiras' mother was an asuri, and so he would have divided loyalties. Brahma, somewhat inexplicably, told Indra that Trisiras was above that, and so Indra invited Trisiras to be the new devaguru. Trisiras took over, and initially, the devas regained their advantage over the asuras. However, this made Trisiras unhappy, since the asuras were his relatives as well via his mother, so he resolved to dedicate his yagnas to both. This started and went undetected for a while, and the battles b/w the devas & asuras were stalemates.
The devas were curious about these developments, while Indra got suspicious. At the next yagna, Indra made it a point to be close to Trisiras and monitor what he was doing. He detected that while Trisiras was doing the main rituals for the devas, he was chanting in undertones something totally different, and Indra suspected that something fishy was going on. When it came time to offer ghee into the fire, Trisiras poured in a spoonful of ghee dedicating it to the devas, making Indra wonder why he didn't empty the whole pot. Trisiras did next empty the whole pot, dedicating it to the asuras. This infuriated Indra, and he drew his sword and cut off Trisiras' heads. The heads took the form of 3 birds that flew away. That yagna was a fiasco.
Twastha found out about Trisiras death and then did some major penances & invoked Vritra, and ordered him to go destroy Indra. In the battlefield, the battle b/w devas & asuras were going on, when Vritra arrived and totally shattered the devas, and Indra was powerless against him.
Indra then approached Brahma for help, and Brahma told him that since Vritra was created by Twastha, he could only be killed by a weapon created from the bones of Dadhichi, whose tapasya alone matched that of Twastha. Since Dadhichi was in debt to the Ashwin twins, who had once saved him from death for imparting to them some vedic knowledge that the devas wouldn't allow, they all asked Dadhichi if he would be willing to give up his life so that a weapon could be made of his bones to kill Vritra. Dadhichi agreed and gave up his life, and w/ his skeletal remains, the Vajra was made.
Indra then led the Devas to battle against Vritra, and this time, he was successful in killing him. This was one of Indra's rare achievements in war.
