DESPEGUE
You are confusing De-rate with Flex. Flex IS a full thrust take-off, only full thrust is assumed at a higher temperature. All performance is calculated for the higher temperature, therefore actual performance will be better than predicted.
Derate is a different story entirely. in the cast of Derate, Vmcg is definately a limiting factor as the thrust is calculated - amongst other things - rudder efficiency, with its limit governing the amount asymmetric thrust it can control.
One advantage of FLEX, is that TOGA can be selected at any time throughout the take-off and first few segments of climb. Derate on the other hand, has limits and conditions (i.e.: speeds must be greater than "XXX" whilst in CONF 2 or 3, etc, etc).
With FLEX Thrust Management, TOGA is a lot like Viagra. You can Fuc& without it, but using it makes you feel better with the only side-effect being that your bitc& screams a little louder.