The 22-year-old started his career at Tranmere Rovers but moved to the Scottish Championship side in 2013.
Vaulks also had a spell in non-league with Workington prior to his switch to Scotland.
He is manager Alan Stubbs' fourth signing ahead of the new Championship season, which the Millers kick off against Wolves on 6 August.
Find all the latest football transfers on our dedicated page.
Rotherham United have signed Falkirk midfielder Will Vaulks for an undisclosed fee on a three-year deal.