Vaakanainen spent the 2017-18 season playing for SaiPa in the Finnish Liiga, accumulating four goals and seven assists for 11 points in 43 games. His plus-eight rating led all SaiPa defensemen. He also competed in nine playoff games, tallying one assist. In 2016-17, Vaakanainen skated in 41 regular-season games for Liiga's JYP, earning a plus-five rating while producing two goals and four assists for six points. He played in 14 playoff games, picking up three assists and a plus-one rating in 14 games. In 2015-16, the 6-foot-1, 185-pound defenseman began his Liiga career with the Espoo Blues when he scored one goal and added five assists for six points in 25 games.

The 19-year-old blueliner appeared in five games for Team Finland in the 2018 World Junior Championship, where he recorded an assist and earned a plus-three rating. He won two medals with Team Finland in the Under-18 World Junior Championship, claiming a gold medal in 2016 while registering one goal and two assists for three points in seven games and a silver medal in 2017, when he totaled three goals and three assists for six points in five games.

The Joensuu, Finland native was selected by Boston in the first round (18th overall) of the 2017 NHL Entry Draft.