Buffering Vref on AD623
If you want the output voltage of the Instrumentation Amplifier to be centered around VCC/2 as precisely as possible then yes, you will need to buffer the VREF voltage.

If you care less about the exact level, for example, you calibrate that level in the ADC (apply zero input, measure output of ADC, store that value and use it as input = zero). Then sure, you can leave out the buffer.

Is this large enough compared to the 10k?

As explained above, there will be an error if the impedance of your VREF source is not significantly less than 100 kohm. 10 kohm is a factor 10 less so the error will be clearly present. If you want a smaller error, use a buffer. It is just a question of what you want/can tolerate.