Student or Learner
What do you mean by 'return one's monastic vows'? Does it mean 'becoming a layman again?
He became a monk in 1997 and returned his monastic vows next year.
The common conception of returned is to give something back. Your sentence confuses me. Perhaps you meant to write: "He became a monk in 1997, but returned his vows the following year." This would imply that he gave up his vows after one year.
How about, "He became a monk in 1997 and renewed his vows the following year."
Depending on situational context, I would substitute returned with renewed.