The VCPD Motorcycle Unit is one of the VCPD's units in 1984. The unit is similar to the patrol unit, as the motorcycle unit also patrols the streets of Vice City. Officers of this unit patrol the streets on VCPD Wintergreens.
The unit doesn't appear in the events of Grand Theft Auto: Vice City because VCPD Wintergreens didn't exist in the game. Another reason for it's absence could be that the VCPD probably dropped the unit.