you can model voltage references like this:
use an ideal opamp and add the parameters for the voltage noise manually: un=flat noise level, uc=1/f corner frequency
op LM336 op0 nii no no un=205e-9 uc=80 # simulate noise of LM336-5 reference
can you post the whole vco model plz. I would like to check the whole model. you might use the wrong opamps as your schematic is not the one for the vco you are debugging...
I had forgotten to include the noise of the LM336 in the noise model because it cannot be easily modeled by LISO. So I thought I would look up its noise to see if I could make an approximation just to see if it will affect the noise levels coming from the model. I found this on a data sheet and will try to find a way to include it in the model.