Constantine and Vladmir: Christian Bros

The conversions of Constantine and Vladmir to Christianity had their similarities and differences.

Constantine had two main reasons behind his conversion:
1) At the battle of Milvian Bridge, when Constantine was about to fight against the Emperor Maxentius, he looked up into the sky, and saw a burning cross with the words "Through this sign conquer" emblazoned on it. After that he won the battle, and believed more in the Monotheistic Christian God than the Polytheistic Roman Gods.
2) His Wife. Although he did gain faith in the Christian religion due to the Cross in the sky, Constantine's wife, who was an ordained Christian, was the one who finally convinced him to convert (to shut her up really).

Now Vladmir also did convert for his wife, but in the sense that he actually converted to win his wife. Anna of the royal family of the Byzantine empire, had rejected marriage proposal after marriage proposal from the rulers of France and Germany, and when the barbarian king of the Russians proposed, it seemed obvious that she would turn him down in the blink of an eye. But Vladimir persisted and converted to Christianity, and Anna immediately agreed to marriage.

So, not saying their faith had nothing to do with the two emperors converting, but their wives/ future wives were probably the biggest deal makers for both of them.