The 23-year-old, who scored 18 goals in 65 starts for Vale, has agreed a two-year contract with the U's.
Manager Chris Wilder told the club's official website: "Sean is a player we have been watching for a while now, as far back as when we were in the Conference.
"I saw him play a number of times last season and he was always a threat to the opposition.
"He can play all along the front line and scored 10 times last season from wide left. We create a lot of chances here and are confident he can score goals for us.
"We are looking for that extra bit of quality in the final third and we are delighted to have Sean here."
