Warne's side need to beat the Bluebirds and hope that Championship relegation rivals Derby County fail to beat Sheffield Wednesday in order to stay up on the final day of the 2020-21 season - on a 'Survival Saturday' which sees all three clubs having an opportunity to avoid the drop.
The Millers have won just once on their past 11 visits to Cardiff but the hosts must do without Vaulks, who is suspended following his dismissal in the 2-1 win over Wycombe on April 24.
Rotherham will still face one familiar face in the shape of ex-Millers loanee Kieffer Moore, who has netted 20 league goals so far this term in a productive first season in South Wales.
But Vaulks, who moved from the Millers to Cardiff in a big-money deal which was worth an initial £2.1m in June, 2019, is absent.
Joking that he is displeased that the Wales international, linked with a summer move to Newcastle United, is not playing, Warne said: "I am not really (pleased) because I know how slow he is, I was intending to get around him, so I am hugely disappointed their weakest player isn’t playing, that has changed all of our tactics...
"We are playing an even stronger team now Will is not going to be in it..
"If he kept coming into our dugout to take his throw-in, he'd be getting so much abuse he probably couldn’t bear it.
"He has had his first child this week as well so he's had a big week and upsetting us wouldn't have been fair..
"(Seriously) He's done brilliant at Cardiff and has been linked to every club in the world. He is a great kid who deserves a great career and I am buzzing he is not playing really!