The story was broken by Mike Reiss on his twitter status and confirmed by Adam Schefter on his twitter with added details - big Vince is returning to New England on a 5 year, $40M deal - $18M signing bonus, $25M guaranteed. As Reiss points out, that makes Wilfork the highest paid Nose Tackle in the NFL (and justly so).
So much for the theory that the Pats won't offer market rate for their top players - they keep a top performer, a steady link to the Superbowl-winning days, a defensive leader, and a genuine all-round locker room influence. They also keep a member of the extended Pats family - Bianca Wilfork. This deal had to be done, and it so it was.