The same place called Vraj is also come to be known as the 1. Country of Surasen and 2) Mathura Mandal after the Pauranic age.
1. Surasen Desa : In the age of Krishna, the father of Vasudevji, belonging to the dynasty of Yadav of the race known as "Chandra Vamsa" - a powerful rule called Surasena - ruled the place. Since the whole place was under his governance it was also called Surasen Desa (Surasen Desh/Surasen Pradesh).
Puranas give another reason for Vraj being called Surasena Desa. This is again taken from Puranas. In the incarnation of Sri Rama a daitya king called Madhu ruled the place. His kingdom was known as Madhuvan and the capital was called Madhura.
He was a devotee of Siva. He had performed violent penances and obtained his "Trisula" (A Trident of Shiva, a spear like weapon with three pearcing parts at the head). He also received the assurance that as long as this weapon will be with him and his son no one would be able to kill him.
After Madhu the Daitya, his son Lavana became the ruler. He harassed rishies and Brahmans very much. The rishies went to Ayodhya and complained to Rama. Sri Rama had sent his brother Satrughan with a large army. When he reached the Palace and challanged Lavana, the latter hurried from the palace without carrying the Trisula. Then Satrughan had killed him. Pleased with his achievement the Devas told him to ask for a boon. He said "Let the people of this place be as brave warriors like me". Those who will always confront injustice and adharma.
Sri Rama then appointed him as the king of Mathura. The people have remained great warriors. Hence the place has come to be known as "Sura Sen Desa".
2. Mathura Mandal :
Even from the Vedic times, this place has assumed the name of Madhura Nagari or Madhupuri - deriving from the demon king Madhu. In course of time it is called Mathura.
About an area of 21 Kosas all around Mathura came under the rule of Mathura. So the circumference of the full area came to 84 Kosas (Distance is measured in Kosa) and is known as Mathura Mandal).
3. Vraj Mandal :
Father of Sri Krishna, Sri Nandaraiji had nine lakh cows in his stables. All together all he Vraj Vasies of area held 72 lakh cows. These cattle used to graze in the nearby 12 forests known as "Vana" and 24 sub-forests known as "Upavana". Hence the area was called "Vraj Mandal".
Puranas quote an incident in this connection. Radha had a companion called "Viraja" One day Radhaji did not pay her usual visit to Sri Krishna. So Sri Krishna wandered along with "Viraja" in the forest (Van in those days were thickly wooded forests bereft of all wild animals. So safe for people and cattle to move in groups). In the meantime Radha arrived there. In order to save herself from the wrath of Radha, Viraja turned into a stream. This stream is latter came to be known as "Yamuna". So the name "Vraj" is attributed to the place adopting it from Viraja.