Protecting microcontroller from inputs rising faster than Vcc
Assuming that Vref is not changing much you can put a grounded capacitor (e.g. 100nF) at the Vref input and connect it via a current limiting resistor (e.g. 10k\$\Omega\$) to the external Vref. The capacitor gives you low impedance. The resistor limits the current in case that Vref > Vcc.

If for some reason that is not an option you can insert a unity gain OpAmp that has a high impedance input and a low impedance output.