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Rockstar have not confirmed the setting of the next GTA but Vice City is as good a guess as any.

In GTA 1 and the GTA III Era (twice), the pattern was Liberty City, Vice City, San Andreas. There is every chance that this will occur in the next GTA too.

In Episodes from Liberty City, the pre-orders get a peice of paper that hints at the next location.

In Episodes from Liberty City theres a radio station called Vice City FM