I don't see what the difference is between turbo wastegate and variable geometry turbo. Both seem to allow some of the gas flow, without turning the turbine, ie. bypassing the turbine, have I understood correctly? The only difference I see, is that the wastegate allows the gas to flow before the turbine, while the variable geometry, after the turbine. Any idea?
The real difference is that a wastegate or dump valve is a way of limiting the maximum boost pressure in order to prevent damage to the turbo itself of downstream parts on the other hand variable geometry is intended to improve performance across a range of engine speeds.
One disadvantage of simple turbos is that they tend to have a fairly narrow effective RPM range which can adversely affect drivability and make it difficult to predict and control torque response. Specifically a wide aperture tends to make the turbo less effective at lower engine speeds and increases lag while a narrow aperture can choke the exhaust flow at higher speeds and limit peak power.
A variable geometry outlet is intended to allow the turbine to be effective across a wider range of engine speeds.