DAC output is completely dependent on Vreff?
Yeah the Vref = 2.5 V and the DAC can produce voltage output not more than the Vref, but if you see the datasheet on page 4 under Power requirement section,



In the table on can see the voltage range is 4.5 and 16.5 V for AVdd and AVss i.e when you configure the DAC this input of analog voltages are used to give an analog output range of

AVss &lt; Analog Voltage Output &lt; AVdd


i.e this is the analog reference voltage you should see when saying that the output analog voltage cannot exceed the reference voltage.

On the DAC you have to apply those voltages which you are mentioning to 'generate', to generate those voltages use a boost convertor, voltage multiplier etc....