Afaik, there are no public API available to check the validity of those certificates (I wish there were).
You can activate an Alarm which will be triggered before a certificate expires.
You can control how far in advance that event will be given with the VCSA advanced setting vpxd.cert.threshold
As stated in Certificate Management Overview the choice depends on what your local requirements are.
In short (and in my opinion), there are no technical reasons or advantages to replace the VMCA, but there might be local policy requirements.
