The 25-year-old, in his third season at the New York Stadium, has been an integral part of the Millers’ survival fight this term.
He has started all but one of United’s 35 league games and netted five goals. Vaulks also scored in the Carabao Cup second round defeat to Everton in August.
Such has been the midfielder’s form that there seems certain to be plenty of potential suitors interested in him during the summer.
Rotherham extending a contract that was due to run out at the end of June means the club will be able to demand a sizeable fee for one of their prized assets, should there be a desire to sell.
Vaulks moved to south Yorkshire from Scottish side Falkirk during the summer of 2016.
He was in the side that clinched promotion at Wembley last May by beating Shrewsbury Town in the League One play-off final.
Manager Paul Warne, meanwhile, is urging Millers fans to get behind his players during a run-in that will see Norwich City, Nottingham Forest, Aston Villa, Birmingham City and Middlesbrough visit the New York.
He said: “We can expect a sizeable travelling support from all of the clubs who will come here for the rest of the season. I like that though, when we have a large away following. I feel it really gets the home supporters engaged.
“It would be great to see a full house for the rest of the season. Our attendances have been brilliant all season, and from experience, when the stadium is full and rocking we always rise to the occasion. Your support really could make all the difference.”