So, I think Erik Nilsson had another daughter (Martha) who was born much earlier (1734). Maybe Erik Nilsson was married before. There were no birth records in Vigåsa that fit around 1734. But, there was a birth record in Vrå in 1732 for a Martha born to an Eric and Kiersten in Ljushult, Vrå (if I'm reading the place correctly):
Ljushult, Vrå was where Erik Nilsson and Segrid's first daughter, also named Kiersten, was born. So, it's possible. I still need to "prove" the connection to Erik Nilsson but think I'm on the right track.
I think you are on the right track!
After a widow or a widower remarried, they often named their first son or daughter after the dead. Seems like that's exactly what happend here. But then you should be able to find a dead Kerstin in Vrå sometime between 1732 and whenever the new Kerstin was born.