Thanks GF.
I meant to say that the thrust can be reduced further, not the Vmcg which, as you point out, has to be valid for the 'rated thrust'.
If we decided to use a full thrust take-off, we would see 'TO' annunciated as the active thrust limit.
Normally we use a reduced thrust, so we see 'D TO' annunciated as the thrust limit, i.e. 'Derated Take-Off' thrust.
If we decided to use TO-1 as our fixed derate (in order to get a lower Vmcg for example), the annunciation would be 'TO-1'. We could then apply the assumed temperature method to reduce further the (already derated) thrust. The annunciation would then be 'D TO-1'
Similarly we could see 'TO-2' if we decided to use TO-2 fixed derate, or 'D TO-2' if we decided to use a reduced fixed derate(!)
In all the above, the Vmcg would be appropriate to the 'rated thrust', i.e. Full thrust in the first example, TO-1 thrust in the second example and TO-2 thrust in the third.
OK, back to my glass of wine!