Thomas, 33, started his career at Arsenal and has signed a 12-month deal after being released by Rotherham.
Free-agent Forrester, 22, has agreed a two-year contract at Vale Park.
Boss Bruno Ribeiro has now signed 11 new players as he continues his major rebuilding job at the League One club.
Thomas and Everton-trainee Forrester join Swedish winger Christopher Mbamba in arriving at Burslem in the last 24 hours.
Find all the latest football transfers on our dedicated page.
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Former Charlton, West Brom and Crystal Palace winger Jerome Thomas and ex-Blackburn striker Anton Forrester have become the latest additions Port Vale's new-look squad.