The striker admitted in his new autobiography he drank a glass of port out of a Lucozade bottle the night before every match last season out of superstition.
He ended the season with 24 goals after firing the Foxes to a shock Premier League win by 10 points.
Vardy has three goals in 10 games this term ahead of Sunday’s visit of Southampton and Ranieri is relaxed about his personal preparation, which also includes three cans of Red Bull and two espressos on the day of a game.
He said: “He took it before Porto? Why didn’t he score!?
“A nutritionist gives them something to eat but after they go home they are professional and can eat and drink what they want.
“For me it’s important to watch on the pitch, when they play and train well I am happy.
“Sometimes the players do something just because ‘ah, I scored a goal and I want to continue’. It’s okay. It’s the doctor who has to investigate and say the port is good or not good.
“Jamie is doing a fantastic career, from non-league to national team. I don’t believe port can do something. It is a glass of red wine. It is okay. For me it is only superstition, nothing more.”
Meanwhile, Ranieri’s assistant Craig Shakespeare remains in talks with the Football Association over his role under caretaker England boss Gareth Southgate.
Shakespeare was part of Sam Allardyce’s backroom staff before Allardyce’s exit on Tuesday, just 67 days into the job, following a Daily Telegraph investigation into corruption.
He only joined the national set-up in August but Ranieri would be happy for him to continue under Southgate.
Ranieri added: “I know they are speaking and Craig is fantastic with us and the FA. He is free to choose so they are speaking and I am waiting.
“If he wants to continue I am happy, if he wants to stay with us I am happy.”
Danny Simpson is available after being dropped for Tuesday’s 1-0 Champions League win over Porto and Nampalys Mendy is fit after an ankle injury but Matty James is out with his own ankle problem.