I appreciate the datasheet is a little confusing but I do not believe it to be incorrect.
Internally the VMID is generated using a simple resistor divider circuit and therefore you should be able to change it using an external source without turning the internal supply off. You should also be able to AC couple a signal onto the internally generated VMID supply.
There is no way of turning the internal VMID supply off without disabling the whole device through the use of the ENABLE pin.
I hope this helps.