2009-10: Mikko Vainonen had one assist in his only game with HIFK's U18 team and was a key defensemen on HIFK's national champion U16 team. Vainonen scored 8 goals with 14 assists in 32 regular season games for the HIFK U16 team. In ten playoff games he scored 3 goals with 2 assists and had 33 penalty minutes. HIFK U16 defeated Helsinki rival Jokerit in a five-game playoff series – winning the last game in overtime.
2010-11: Vainonen played a handful of games for HIFK's U20 team, spending most of the season with the club's U18 squad, and represented Finland at the U17 European Youth Olympic Festival and World Hockey Challenge tournaments. He scored 1 goal with 2 assists in eight games and was plus-one with 12 penalty minutes – averaging 17 minutes a game with the U20 team. He played in 4 of 5 games in the playoff series with second-place Espoo and was an even plus-minus with no points and 6 penalty minutes. He played 24 games for HIFK U18, scoring 4 goals with 10 assists and 24 penalty minutes. He played in 1 of 4 playoff games in the series with Karpat and was scoreless with no penalties. Vainonen scored 1 goal in three games as a team captain for Finland at the EYOF and had 2 assists and 10 penalty minutes in five games at the WHC.
2011-12: Vainonen made his SM-Liiga debut – appearing in eight games for HIFK – and was a key member of HIFK's Finnish national champion U20 team. He was a team captain for Finland at the U18 World Junior Championship. Vainonen played six early season games and two more in February – averaging just under 11 minutes of ice time – and was minus-one with 2 penalty minutes for third-place HIFK. He did not play in the SM-Liiga playoffs. Vainonen was the second-leading scorer amongst defensemen behind Jori Pakkanen on the U20 team – finishing with 7 goals and 11 assists in 38 games; and was plus-seven with 44 penalty minutes. HIFK U20 defeated Jokerit in the playoff finals after finishing fifth in the regular season. In ten playoff games Vainonen had 1 assist and was an even plus-minus with 12 penalty minutes. Vainonen had 4 assists and was an even plus/minus with 8 penalty minutes in seven games at the U18 WJC for fourth-place Finland. He was selected by Donbass Donetsk in the second round (68th overall) of the 2012 KHL Draft. Vainonen was invited to the NHL Draft Combine and was ranked 11th amongst European skaters in Central Scouting's final rankings prior to the 2012 NHL Draft.
Vainonen is a big defensive-defenseman that plays hard and can make a good first pass to start the transition game. He keeps things simple and his game is very effective. He needs to improve his skating speed, reverse pivots and balance. The young defender will have two years of development at the junior level to learn how to play with more physicality and use his big body to box-out the opposition around his net. The Finnish born player will need time to adjust to North America and its style of game.
Vainonen has chosen to come to North America early in order to play his junior hockey in the OHL for the Kingston Frontenacs. He should play on the penalty killing unit and could show some offensive upside as he becomes more comfortable with the smaller ice surface and rougher style of play at the junior level. The Finnish defenseman could represent his country at the 2013 WJC tournament.