Hi ddarko,
The Vlogic pin of the AD5684 is the “Digital Power Supply” which powers up the digital block of your IC. It also sets on what level your digital communication line will be. The usual Vlogic levels are 1.8V, 3.3V and 5V but it can be of any value between 1.8V and 5.5V. The Vdd pin powers up the analog parts of the IC. Since your microcontroller is powered with a 3.3V power supply, it is logical to set the Vlogic to 3.3V as the microcontroller will be the one to handle the digital communication to the device.
On the other hand, if you use a reference of 2.5V and a gain of 2, and connected Vdd to 5V, you will still get an output between 0 to 5V.
Regards,
Mark