Many case the VDAC outputs are used as just refernces for low impedance inputs. in such cases an elaborate output buffer might not be required. you can achieve more channels using external capacitors for the hold and analog pin hardware switches for the sampler. samplers being on pins mean more multiplexed channels.
The switching frequency here would be limited by the RC constant of the series impedance of the pin switches and external capacitors. The Buffer on the VDAC output takes the VDACs output impedance out of the equation..
Not sure if just using a capacitor is a good idea if the loading is low impedance, you need a larger cap. to hold the value, not to mention the "ripples" due to charing and discharging of the capacitor. It may improve with a faster switching time, but would use a standalone DAC if a stable reference is neede.