It's always a question, just what is the "op amp" - the
differential to single ended amplifier itself, or the two
sides of the PTAT and the output voltage? And do you
care about the op amp's stability in isolation at all, or
only the net output stability of the block?
I would be inclined to work mainly from the VDD node,
apply voltage step and look for the output node of
interest's overshoot / undershoot, samping and PSRR.
These are what matter to the function. Of course there
is a likelihood that other things are more important to,
say, a professor looking for closed form analysis or a
demonstration of a particular design methog.