Hi, Malinovskij,
Welcome to e2e, and thanks for your interest in our devices!
Several places in the d/s, it refers to VREF = VCC/2, so this means VREF will 100% track any changes in VCC. It is most likely generated via a resistor divider, so that output impedance of that resistor divider (unfortunately, I don't know what that is) combined with the recommended capacitor on the pin will give you some immunity to ac changes on VCC, but dc changes on VCC will cause VREF to track accordingly.
-d2