Part of Greece, as Archipelago or coastal regions, is possible at least for religion. It happened IOTL under Latin occupation and administration.
Whole Greece, however, even if Venice could have conquered all of it, would be close to impossible : religious (in a region that rejected Roman Catholicism with all the energy it got), administrativly, and in matter of prestige, it was simply too great, and the benefit in imposing a romance language would be too unimportant to go trough all this sort of troubles.