MFB Analog filter with Reference voltage transfer function
A DC bias voltage Vref is applied (instead of grounding the non-inv. input node) to establish a suitable bias point at the opamp output that is NOT zero volts. This is necessary in case of single supply only.

On the other hand, the transfer function of the whole circuit concerns the ac response only - independent on the supply voltage and the chosen dc bias point.

Hence, there is no influence of Vref on the transfer function.