Benfica finished third in Portugal's Primeira Liga last season after 11 draws left them behind both Porto and Sporting.
"Playing Benfica is a good opportunity to switch our focus," added Vrancken, who has no fresh injury concerns.
"If we can get a good result it would be a huge achievement, but we have to stay humble, fight together in every moment of the game and stay disciplined.
"Against Sturm Graz and Aberdeen we lost the game in a few moments. If you do that against a side like Benfica, they will punish you.
"When you get the chance to play in Europe against Benfica in their own stadium, you don't need any extra motivation.
"If you do need extra motivation, that's not a good sign. Everybody who steps into that stadium will be motivated. We just have to stick to the plan, give ourselves a chance and create something."
Portuguese forward Claudio Braga will make his first appearance at Benfica's Estadio da Luz and knows exactly where Hearts must improve if they are to remain in the tie.
"We need to make fewer mistakes and that's the big difference from the previous games," he said.
"Of course we know they are a big team and stronger than us on paper.
"But nothing is impossible in football and we hope we can cause them problems."