Lampard comes off contract in June.
Vialli 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."