Thibault, who played for Montreal from 1995-98, recorded his 38th career shutout -- his first since March 14 for Pittsburgh in a 3-0 win in New Jersey.
"It's always special any time I get a chance to play against Montreal, even more here, and I was happy that Lindy (Ruff) gave me a chance to play tonight," Thibault said. "I think the guys knew that this game meant something special to me, coming back home. The guys just put up a great effort tonight."
It was just his third start of the season for Buffalo, which signed him as a free agent in the offseason.
"We did research on him and we were looking for a veteran guy that could be positive and support a guy that he knew was going to play a lot," Ruff said. "We felt we brought in a good guy and his work ethic and attitude has been tremendous. I was thrilled for him tonight."
Spacek scored on a power play midway through the second and Vanek made it a two-goal lead later in the period as the Sabres extended their winning streak to four, including three against the Canadiens.
Jochen Hecht scored into an empty net with 26.3 seconds remaining as Buffalo beat Montreal for the second night in a row.
The Sabres improved to 3-2 in the season series against their Northeast Division rivals following Friday's 4-2 win.
Cristobal Huet stopped 32 shots for Montreal, which has lost five of eight since getting off to a 9-3-3 start.
"(Thibault) made some nice stops, he deserves the shutout but we didn't get nearly enough quality chances on goal," Canadiens left wing Christopher Higgins said. "We were just turning the puck over way too much and not getting the puck deep and against that team, they're good on the transition, they feed on our turnovers and we made a lot of them tonight and made them look pretty good."
Buffalo defenseman Henrik Talinder left with a shoulder injury in the second period. He did not return.
"Right now, we're looking at days," Ruff said. "We'll get an update probably when we get back."
Spacek scored his fifth goal 9:38 into the second, just nine seconds after Montreal's Tom Kostopoulos was sent off for hooking.
The Sabres defenseman scored on a tight angle shot from the right side of the net after he collected a rebound of Bryan Campbell's point shot.
"It's key to get a lead at any time," Campbell said. "You shoot the puck and find some holes."
Vanek made it 2-0 with his sixth goal at 13:54.
- Thibault had one shutout with Pittsburgh last season, 28 with Chicago from 1998-2004, seven with the Canadiens, and one with Quebec in 1994-95.
- Sabres G Ryan Miller has a 2-2 mark against Montreal this season despite maintaining a 1.76 goals-against-average in four starts against the Canadiens.
- Huet fell to 2-2 against Buffalo this season. Rookie G Carey Price stopped 28 shots Friday in his first career appearance against the Sabres.