Hi,
The datasheet is not clear on that.
I would assume if you put between 2.25 and 2.75 on Vdd, the core is internally run off a 2.5V bus.
I would assume if you put between 3.00 and 3.60 on Vdd, the core is internally run off a 3.3V bus.
This goes in line with the Vddo voltages, it is selectable by the user.
There doesn't seem to be a part number to designate any particular core voltage.
Have you googled si5351a arduinoIt returns many examples of how the IC is used with arduino and module PCBs.
Tom...