Originally a forward, Vadnais was shifted to defence in his final year of junior hockey with the Montreal Jr. Canadiens.

[2][3] On February 23, 1972, Vadnais was acquired by the Boston Bruins[3][4] in an attempt to bolster their blueline for a Stanley Cup run.

[5] Vadnais was involved in the November 7, 1975 blockbuster trade that sent him along with star forward Phil Esposito to the New York Rangers for Jean Ratelle and Brad Park.

[6] Along with Wayne Cashman of the Bruins and Serge Savard of the Winnipeg Jets, he was one of players from the NHL's pre-expansion Original Six era whose NHL career lasted the longest; the Bruins and Jets both made the 1983 playoffs, extending Cashman's and Savard's careers into that post-season.

Vadnais played 1,087 career NHL games, scoring 169 goals and 418 assists for 587 points, as well as adding 1,813 penalty minutes.