Why are you trying to assume an outlet velocity profile? What is the purpose of your simulation?
About the difference between outflow and outlet vent:
Outflow - Outflow boundary conditions can be used at the outlet of open channel flows to model flow exits where the details of the flow velocity and pressure are not known prior to solving the flow problem.
Outlet vent: Outlet vent boundary conditions, as the name indicates, are used to model an outlet vent with a specified loss coefficient and ambient (discharge) pressure and temperature.
You should be able to read more about these BCs on the ANSYS Fluent Users guide (Section 6.3 'Boundary conditions', R18.2).
I hope this helps.