Vahtre ended his studyings in University of Tartu in 1955 and worked there until 1993. He studied Estonian agrarian history, medieval chronicles, cultural history, Estonia's christianization in 13th century and St. George's Night Uprising. In 1973 he got PhD. In 1989 Vahtre reestablished chair of Estonian history University of Tartu and was its chairman until retirement in 1993. Until his death he was most important historian in Estonia, who was chief editor of sequence of Estonian history (he edited volumes IV and VI) and also book of Estonian Chronology (first edition 1994, second 2007).
His older son Silver Vahtre is artist, younger son Lauri Vahtre is politician and historian.