Now that the EU has taken the US off the "Safe List", it is up to each individual State member to implement the restriction or not. I have been unable to find any information on the Internet on what Greece plans to do. Does anyone now if Greece plans to allow vaccinated US citizens?
Greece is allowing Americans in and have not said anything about changing their rules.
Just saw this information from https://www.schengenvisainfo.com/news/greece-prolongs-domestic-flight-requirements-for-another-week/
You need to be sure you check official information for/from Greece. https://travel.gov.gr/
Are you vaccinated and do you have a CDC certificate (white card)? If yes, that, and the locater form, are all you need right now to get into Greece.
It's also only what you need to take a domestic flight.
As for the future, no one knows. But since Greece was one of the first European countries to open, I would be surprised to see them close. In fact, I have not heard of any EU country changing their entry rules for Americans.