Does code require a new bath vent fan to be on a dedicated circuit?

No, the bath vent fan does not need a designated circuit. If this is a new installation install a three wire cable to the fixture from a two gang switch box. Cover plates for two gang boxes can be bought with a blank on one side and a switch on the other. At some time in the future a lighted fan enclosure may be installed. This will allow the light and fans to be operated independently.