Hi,
"I have uC pins 3.3V and 5V which I use as Vref now. But what could be better to use in this case for Vref? A voltage regulator which regulates from opamp supply or a voltage regulator from uC supply? What component could Vref be?"
Only you can know how precise you want/need Vref to be across expected temperature range and supply voltage fluctuations/ripple. I only understand you need a mid-supply reference, I do not know how fussy you are or how precise the circuit must be. I thought MCU regulated output voltages are rather imprecise and quite... let's say basic. Check the MCU datasheet to see how much the nominal '3.3V' is actually 3.300V (!) and how much it may change with temperature.
There's an argument for using the supply rail or MCU regulated output voltage and a resistive divider as Vref as then everything is moving around/up and down at the same level (unless there are terrible spikes on the supply line). This, again, only you can know what the circuit needs with regard to a reference voltage - precision or supply voltage synchronicity between components.
I wouldn't use a voltage regulator as a reference myself, especially as some people even use a voltage reference IC in an adjustable voltage regulator's feedback loop to fine tune the voltage regulator's 'imprecise' output voltage... In theory, at least: voltage regulator = coarse regulation; voltage reference = fine regulation.
TL431 is a simple to use adjustable part, but it drifts somewhat for my liking depending on what I am hoping to achieve. e.g. TL431C has an initial voltage at 25ºC = 2.440V min, 2.495V typical, 2.550V maximum, it can drift an additional 4mV to 25mV with temperature changes. I was surprised to see the LM4041 is far 'worse' than the TL431 with regard to temperature drift. There are many other voltage references that are far 'stiffer', you just have to do a parametric search.
If you don't feel like reading a lot of datasheets to see which voltage reference has the lowest ppm/ºC and initial tolerance or what load it can handle (irrelevant point as an op amp biasing device which will have tiny bias currents), or it's not important for your needs, try the TL431 (~2.5V), or TLV431 (~1.25V), for example - easy to use and importantly, the Spice, etc. models are widely available.