The 24-year-old, who agreed terms before he went on holiday, will now stay at Vale Park until the summer of 2015.
Williamson said: "I am absolutely buzzing for it, obviously it was a great season last season, I've just got to try and push on from here really.
"Moving up to League One was a big factor, I'm quite settled here now and I've got a good relationship with the boys."
The front man scored nine goals for Micky Adams' side last term as they won promotion from League Two and he is ready to battle Tom Pope and Lee Hughes for a place in attack next season.
"Ideally I want to start more games," he added. "Last season I didn't start many games but I still managed to score nine goals which was okay but I want to be starting, hopefully if I start more I'll score more.
"There is always going to be competition for places no matter where you are so hopefully I will just get a good pre-season under my belt."