Sinnott said: "We observed him in the two games he played for us and liked what we saw. He has come from a good club and has got a good grounding there.
"I think he showed in the two reserve games what he's about. He is a hard working player and has an eye for goal. He will cause people problems, he has that type of mentality."
Thompson is also prepared for the challenge ahead at Vale Park and is hoping to prove Sinnott was right to sign him.
He said: "I would like to thank Port Vale for bringing me here and giving me a chance. Hopefully I can repay them by scoring a few goals and help take the club back to where they belong."