Stepan was scheduled to have surgery Friday night. His status for Game 4 Sunday at Madison Square Garden (8 p.m. ET; NBCSN, CBC, RDS) won't be known until Saturday at the earliest, according to Vigneault.
The Rangers lead the Montreal Canadiens 2-1 in the best-of-7 series.
Stepan was injured when Montreal Canadiens forward Brandon Prust hit him 2:45 into the first period. Stepan left the game and had X-rays taken, but they did not reveal the injury. He returned during a television timeout at 8:15 of the first and resumed a normal shift.
Stepan played 24 shifts totaling 17:46, including 22 shifts for 16:52 after returning to the game. He delivered a hard hit on Montreal defenseman Alexei Emelin on his first shift back in the game and assisted on Chris Kreider's game-tying goal in the final seconds of the third period.
Vigneault said he did not become aware of Stepan's injury until Friday morning.
Prust was suspended two games Friday by the NHL Department of Player Safety for what is being called interference on the play. He was not penalized for the hit by the on-ice officials in the game Thursday.
Stepan is tied for second on the Rangers in scoring in the Stanley Cup Playoffs with 11 points in 17 games.
The Rangers already were missing one of their top four centers with Derick Brassard sidelined the past two games with an upper-body injury that he sustained early in the first period of Game 1.
Vigneault said Brassard remains day-to-day.
The Rangers also will be without forward Daniel Carcillo, who received an automatic 10-game suspension Friday under Rule 40.3 Physical Abuse of Officials -- Category II. Carcillo physically applied force to linesman Scott Driscoll while being escorted to the penalty box following Carcillo being assessed a minor penalty for charging at 5:51 of the first period of Game 3. Carcillo also was given a game misconduct.
J.T. Miller and Jesper Fast have been the Rangers' extra forwards in the series. Both have played three games in the Stanley Cup Playoffs, but the last time either was on the ice was Game 3 of the second round against the Pittsburgh Penguins.
"Players that we have on our team if we need, they should be ready to play," Vigneault said.
"That being said, we'll have a better idea [Saturday] of what's available to us and make the right decisions."