Since your DAC is only 8 bits wide, you should concider sampling 8-bit values only. Additionally you might use a fixed sample rete generated with a timer or PWM.
Re-playing the sampled values with DMA now is easy, just set up your DMA and transfer with the sample-rate from above your data to the VDAC8.
When sampling more than 8 bits (let's say 12) this could be VERY challenging. As you see, only the upper 8 bits can be used and must be transfered to the VDAC8. Thus given the 16 bit Data has to be shifted left for 4 bits and just the upper Byte needs to be transfered.
This can be done with a DataPath programmed to do exactly what you need. But this is not as easy as it sounds. here is something to read or watch to get a glimpse what DataPath is https://secure.cypress.com/?app=search&searchType=keyword&keyword=datapath