Vailanu is the second Wasps player Warriors have recruited for next season following tighthead prop Jack Owlett.
They join Scotland international wing Duhan van der Merwe (Edinburgh), England scrum-half Willi Heinz (Gloucester), Wales hooker Scott Baldwin (Harlequins), scrum-half Will Chudley and tighthead prop Christian Judge (Bath) as newcomers.
“Sione is really powerful and explosive,” said Worcester boss Jonathan Thomas. “I have been an admirer of him as a player from his time with Saracens and since he joined Wasps.
