The 19-year-old had signed a contract extension in January which ran until the end of last season, before being offered a fresh deal.
Plymouth manager John Sheridan has said that Vassell is a good prospect.
"With him, it's belief in himself, more than anything. He trains with us and I just think he's a bit insecure when he's in and around the first team," Sheridan told BBC Sport.
"But I've told him that he needs to grow up because I've seen something there.
"I think he was surprised, but I think there's something there, some attributes that could make him a good player."
Vassell has made eight appearances for the Home Park club.