Read analog signal over the SPI bus using MCP3201 ADC and Arduino Uno
It is possible to use the SPI bus when the Vref of MCP3201 ADC is 2.5V?


Of course it is. VREF is purely a reference voltage for taking measurements. It has no other role. 

If you look at the functional block diagram from the datasheet:



You can plainly see that VREF is only connected to the DAC that is used as part of the SAR feedback loop. Everything else is fed from VDD