The success of controlling the dynamics with velocity, has probably more to do with the way your keyboard produces the velocity data. Non weighted keyboard tend to be too easy to produce high velocity values, and so are hard to control the inbetween bits.
But, assuming it's not a keyboard velocity generation thing, it's important to know where the sample crossover point are, the choir has 4 velocity layers, so be aware of which tonal layer you need. Don't play softly (low velocity) and then turn the volume up to compensate, and the opposite applies too, don't play loud (high velocity) and turn the volume down - you need the right tonal character for the level you are at.
I woulld always help the dynamics along with EXPression (CC11), and I also tend to use the FILTER (CC24) as well, maybe not quite so much with the choir, but using the filter as a dynamics control create the lower harmonics at lower level thing, that most instruments adhere to.
The CC data will need to de drawn on after your performance in your DAW (Cubase in my case), as it generally needs to be gentle curves. If you do have any spurious notes that are the wrong level, you will have to edit the velocity value to correct this. Don't let CC and velocity values put you off, the numeric values can sometimes look odd - use your ears, as that is all that really matters.
And don't be afraid to use as many articlation choices as you have available to help it sound convincing - you will not/cannot achieve this detail in realtime - it has to be done afterwards.