You can do VCO simulation with static DAC at most.
If you want to care dynamic effect of DAC to VCO such in Delta-Sigma-Fractiona-N Synthesizer, you have to use Transient Noise Analysis.
Use correct terminology, there is no "beat frequency", if you use Cadence Spectre.
See
https://www.designers-guide.org/Forum/YaBB.pl?num=1268969030/7#7
VCO is a autonomous circuit.
On the other hand, DAC is a driven circuit.
Common divisor between two frequency can never exist.
Quasi-Autonomous HB Analysis can not apply to DAC + VCO.
And you can not understand Pnoise analysis is no more than small signal noise.
If your concerns are small signal noise,
first evaluate noise PSD of DAC alone using Shooting-PSS+Pnoise,
then apply this noise PSD to ideal voltage source,
do HB-PSS+Pnoise simulation of VCO with this ideal voltage source having noise PSD instead of DAC.