Left-arm quick Vaas, 31, has taken 269 Test wickets for his country since making his debut in 1994 and in One-day Internationals he was also a member of Sri Lanka's 1996 World Cup-winning team.
His signing remains subject to official clearance from the Sri Lanka Cricket (SLC) but he is expected to arrive at New Road for the start of the season in April.
Worcestershire director of cricket Tom Moody, the former Australia international, said: "Chaminda's career record, mostly on wickets in the sub-continent, speaks for itself and it is our belief that his style of bowling will ideally suit the playing conditions during the early part of the season.
"As a fast medium left-arm swing bowler, he will complement the rest of our attack."
Vaas added: "I have always enjoyed my cricket in England and very much look forward to this opportunity with Worcestershire.
"I know how ambitious the County is and I look forward to playing with so many quality players."