Prerna came back from a set down to win her third successive title here with a 4-6 6-4 6-3 victory over Karman Kaur Thandi.
For Vishnu, who last played the tournament in 2011 when he lost to Yuki Bhambri in the final, it was a welcome return to the championship as he ended a fantastic run of Siddharth Rawat in the final.
Vishnu won the opening set comfortably before Siddharth came back strongly in the next. But Vishnu eventually snatched the victory 6-2 4-6 6-3.
"I have been to the final of this tournament twice before and the last time I was here, I lost in the final so I was determined to win this time. It's always good to win the Nationals here," Vishnu said.
Prerna had a tough start against Karman who played solid in the first half of the match. Karman broke Prerna in the fifth game of the first set and it was difficult for the defending champion to break back against the strong serves of Karman.
Prerna fought back in the second set and broke her opponent in the 10th game to take the match into the decider.
She brought all her experiences into play and forced Karman to make mistakes. Eventually, Prerna won the set 6-3.
Young Siddharth had virtually no answers to what was going on and he looked a pale shadow of how he played in the last few rounds. Gradually with time, he settled down and looked more comfortable while facing Vishnu's big serves.
In the boy's under 18 final, Vidit Vaghela fought a tough battle before prevailing over Dalwinder Singh 6-4 2-6 6-4.
