Messi was thoroughly overshadowed at the Allianz Arena a week ago as Bayern romped to a 4-0 win in the semi-final first leg, leaving Barca needing to stage the greatest fightback the competition has seen to make the final at Wembley.
The Argentinian has been troubled by a hamstring injury, which appeared to affect him in Munich, but he came off the bench and scored against Athletic Bilbao at the weekend.
Vilanova said: "The more in form he is the better our chances of getting to the final, but we can't heap this kind pressure on him, we have to try to help him.
"A player as good as Messi, at this point in the competition, will have things to say in tomorrow's [Wednesday's] match."
Bayern have won their last 10 games in all competitions and are now firm favourites to win the Champions League, but Vilanova says his team believe they can come back.
Barca won 4-0 against AC Milan at the Nou Camp in the last-16, although that was to overturn a 2-0 first-leg deficit.
Vilanova said: "We believe we can fight back, it's our obligation and we're convinced (it's possible).
"We know that it will be very difficult, but we are Barca and we can never give up on a tie, especially when we have 90 minutes of play at home.
"We have to hold our heads high if we are eliminated. We cannot give up, we need to make our fans proud."
Defender Gerard Pique added: "The only team that can overcome a 4-0 to Bayern Munich is this club."
Vilanova also emphasised the need for patience on Wednesday, pointing out Bayern only led 1-0 at half-time in the first game.
And he called on the Nou Camp faithful to play their part and create an atmosphere every bit as partisan as Bayern enjoyed last week.
"If we are going to mount a comeback we need our fans. Both Bayern and Borussia (Dortmund, who won 4-1 at home to Real Madrid in their semi-final first leg) had extraordinary atmospheres last week and that carried the teams forward.
"We will do our part so that fans find it easier to do theirs."
Barca will definitely be without defender Javier Mascherano through injury, while midfielder Sergio Busquets is a doubt with a slight groin problem.