Yaropolik might not have considered Vladimir a threat, since Vladimir was the son of their father and their father's housekeeper.
The ruler of Norway, Haakon Sigurdsson, was a relative. Haakon helped Vladimir (who was only 21) to gather a Varangian army and defeat Yaropolik, taking over Novgorod. He was crowned 11 June 980.
On his way to defeat his brother, he sent to the Prince of Polotsk (in Belarus) asking for his daughter Rogneda's hand in marriage. Rogneda refused to marry the son of a housekeeper, so Vladimir diverted to Polotsk, conquered it, kidnapped Rogneda, killed her parents, and used Polotsk as a fortress base to capture Kyiv.
During his reign and expansion of his father's territory, Vladimir lived the life of a staunch pagan, erecting shrines to gods and enjoying 800 concubines. He was keenly aware of the religious beliefs of the various tribes and areas he was conquering, and wanted to ensure loyalty to himself by showing reverence for their deities, at one point building a temple to six different gods worshipped by different groups.
This attempt to exploit the gods for his own purposes was overt enough that people resented it. Vladimir, fascinated with the power religion had, sent emissaries to explore the major religions of the world: Islam, Christianity, Judaism. Islam's prohibition against alcohol and pork made it unsuitable, and Jews' loss of Jerusalem showed that God did not favor them. His people returned having been most impressed by the grandeur of Constantinople and its religious rituals.
There are two different versions of what happened next: the local sources that say Vladimir decided Christianity was preferable, and Arab sources that offer a more cynical reason for his conversion. I'll give you the details next time.