Lampard is out of contract in the summer but is expected to consider a new deal if Jose Mourinho returns to Stamford Bridge as manager.
However, Vialli, who won five trophies whilst in charge of Chelsea, believes the time is right for Lampard and the Blues to part ways.
He told Sky Sports News: "In my opinion it's probably the right time for them to say goodbye to each other and leave on good terms.
"Frank has a fantastic opportunity to try something new, perhaps in Los Angeles or somewhere else in America, have fun for the next three or four years and make some money.
"Then one day he could perhaps come back to Chelsea as a hero. I really hope he scores a few more goals to become their top-scorer in the history of the club."
Vialli expects Mourinho to return to Chelsea this summer and believes it would be a good appointment by the club.
"I don't know whether it's a done deal but I expect Jose to be in charge next season," he said.
"I think he is a good choice. He is unbelievably motivated; he's got a passion for English football and for Chelsea. He is a winner."