[1] Vaakanainen earned a regular role within the JYP Jyväskylä blueline in the 2016–17 season, appearing in a career high 41 games, in matching his offensive output with 6 points.

On 20 October, Vaakanainen was called up for the first time by Boston[6] and made his NHL debut that night in a 2–1 loss to the Vancouver Canucks.

[8] He was assigned to Providence and finished the regular season with 30 games played in the AHL, scoring four goals and ten points.

[10] He made his NHL season debut on 12 November in a 5–4 shootout loss to the Florida Panthers, replacing the injured Torey Krug in the lineup.

He was paired with Derek Forbort initially, but also saw time with McAvoy after he returned from injured and fellow call-up Tyler Lewington.

[9] On 19 March 2022, Vaakanainen was involved in a trade by the Bruins that sent him along with John Moore and multiple draft selections to the Anaheim Ducks in exchange for Hampus Lindholm and Kodie Curran.

[9] On 23 July, Vaakanainen as a restricted free agent was re-signed by the Ducks to a two-year, $1.7 million contract extension.

[20] During pre-season for the 2022–23 season, Vaakanainen was hospitalised after he crashed into the end boards in an exhibition game against the San Jose Sharks on 30 September.

[21] Missing the first 22 games of the season, Vaakanainen returned to the Ducks after a one-game conditioning stint with their AHL affiliate, the San Diego Gulls.

[9] He suffered an injury in a game versus the Vegas Golden Knights on 13 November and was placed on injured reserve on 4 December.

[26] On 6 December 2024, Vaakanainen was traded to the New York Rangers, alongside a fourth-round pick in 2025, in exchange for defenceman Jacob Trouba.