Bringing up old topic just because this became personally relevant, and perhaps useful for someone else as well.
You can do an aoforce calculation simulating the frozen state of the atoms by setting their mass to "infinite". So before starting aoforce, edit the control file and add very heavy masses to the frozen atoms. For example, assuming carbons 1 and 2 were frozen during optimisation:
c 1-2 \
basis =c def2-SVP \
jbas =c def2-SVP \
mass =99999999999.9
c 3-6 \
basis =c def2-SVP \
jbas =c def2-SVP
You'll get the same results as with NumForce and -frznuclei, ignoring numerical noise. The inactive modes will have (almost) zero frequencies. NumForce with -frznuclei sometimes produces some unphysical very high frequencies, these will also be "zero". The total number of vibrations, including translation and rotation, will be 3N.