Dithered VDAC is implemented by using DMA and DAC. So it does not have any hardware input register to access it through the external DMA. When the 12 bit input code is written by using API, the 12 bit value will be separated into four 8 bit data and using DVDAC DMA, the 8 bit data is transferred into the internal DAC. So there is no hardware register for 12 bit data to be written. If you would like to use external DMA to write the 12 bit data, again firmware is being used to separate 12 bit data into 8 bit data.
Let me know if this answers your query.