How can I keep this MOSFET in cut-off when Vcc > 5.5V?
First of all I would not use an op-amp as a comparator, they are not really designed to work that way. 

Instead find a suitable open collector true comparator of equivalent performance and wire it this way.







simulate this circuit &ndash; Schematic created using CircuitLab

It may also be a prudent to add a little positive feedback to that circuit to add some hysteresis. Otherwise, when the trigger is hovering around the reference voltage that output is going to turn on and off very rapidly due to any ripple or noise that is in the circuit.







simulate this circuit