The Whitecaps (8-5-5) secured their first Cascadia Cup win last week thanks to an emotion charged 2-0 win over the Seattle Sounders in front of 22,500 fans at BC place.
The result meant just as much to the players as it did to the fans, with some 'Caps players savouring the moment by dancing in front of the Southsiders supporters section afterwards.
But with more than a week of rest ahead of a date with the Fire (6-8-3), Rennie has used the time to keep his players grounded as the 'Caps look to extend their unbeaten streak to a club-record six games.
"That is a big concern, making sure that our focus is right," Rennie said when asked about the lingering effects of a historic win.
"Obviously the atmosphere was fantastic last week and it was really lively, so the players have to make sure they come with a real intensity (Sunday) because it might take a little bit longer for the game to get going.
"So we have to make sure that we are on top of it and very focused and that's something we've got to make sure we're ready for."
A fast start against the Fire — who saw their own six-game undefeated streak end last weekend in a 2-1 home loss to Sporting Kansas City — could be key as Chicago have given up five goals in the opening 15 minutes of games this season.
Whitecaps striker Kenny Miller scored after just four minutes against Seattle, while the Fire trailed Sporting 2-0 after just 10 minutes.
The Eastern Conference outfit have also conceded the first goal a staggering 14 times — the most in MLS — and have compiled a 4-8-2 record from those encounters.
Rennie said there were some similarities between the Sounders and Chicago and urged his team to push forward from the kick-off.
"I think they are similar in their set-up to Seattle so we have to play the same and really get on top of them from the start and if we can get an early goal that would be fantastic for us," he said.
Besides the shaky Chicago defence, Vancouver will be looking to put plenty of pressure on goalkeeper Paolo Tornaghi, who is filling in for first-choice custodian Sean Johnson (international duty with USA at the Gold Cup).
Tornaghi has started the past two games and conceded two goals in each of them. In late March, the Italian was in net during a 4-1 drubbing by Chivas USA.
Said Rennie: "We want to get on top of their whole team and the fact that it's not their starting goalkeeper may be a little advantage for us."
The Whitecaps are set to field the same starting lineup that took the pitch against Seattle, with Johnny Leveron and Brad Rusin playing in central defence once again.
Even though new Danish recruit David Ousted is now eligible to play, goalkeeper Brad Knighton will keep his place in net after his brilliant shutout against Seattle was rewarded with MLS Player of the Week honours.
"The lineup picks itself pretty much when you win," Rennie said.
Rusin, 26, has started the past two games after overcoming a calf injury and he is enjoying his role alongside Honduran Leveron, despite the usual language issues.
"For the most part it's been good," the American said. "We've played against two quality teams in Sporting Kansas City away and Seattle here and to be fair we got two pretty good results.
"Ultimately it's just about being confident and going out there and doing what we do, doing what I do and of course Johnny just doing what he does — controlling the ball and playing long — things like that so I think we make a good pair."
Playing in front of the in-form Knighton — who has kept clean sheets in two of the past three games — has also helped.
"It definitely gives us confidence in front of him and just knowing he's gonna be there," Rusin said.
Rennie is satisfied with the output of the Rusin-Leveron tandem but the imminent return of veteran Andy O'Brien will create some competition, which doesn't faze the Indiana native.
"You always want competition for positions," Rusin said. "It makes you a better player, you've got to be honest with yourself and you can't take days off even at training so I think it's good for all of us."
The spotlight will be on Rusin and Leveron this weekend with top marksman Mike Magee coming to town.
Since returning to his hometown club from LA Galaxy during the season, Magee has netted five goals in six games but Rusin won't be putting too much emphasis on the match-up.
"It doesn't really matter who I play against," he said. "I just think that's how all of us are. If we come out here and play our game and worry about ourselves, I think we'll get the job done."
Notes: Fire midfielder Daniel Paladini will miss the game through suspension. He has two goals and three assists this season. . . Jamaican international Shaun Francis is set to make his debut for Chicago this weekend. The left-back will replace the suspended Gonzalo Segares. . . Chicago captain Logan Pause (back) could return after sitting out the past two games while fellow midfielder Patrick Nyarko (hamstring) is confident he will be right for Sunday.