if you believe having the external Vref pin, is a better option, I can change the design to STM32f103VXX, although I don't use I/Os and it is significantly more expensive.
Still, nobody replied that using two external resistors can affect the quality of the TL431 voltage reference, in face of temperature variance, or not.
if it does not, I still can select the STM32F103C (48 pins) and make the 3.3V for the VDDA separately using reference and connect the VDD to a linear 3.3V supply.