My intent is for all slave sites to be in the same folder.
I'm having trouble with Symbolic Link setup for CentOS/Plesk 9.
I have tried vhosts.conf in the conf directory of the domain root, to no avail.
But if my intention is the run all the slave sites from within the master directory setup is it even needed for me to allow symbolic links?

No this is not mandatory.
You can also setup you plesk to specify the same "documentroot" for all the domain and sub-domain.
See also the user manual chapter 3.2.6 and the FAQ /faq#sec-76
that give a sample on how to configure plesk.
This is example provided by one of my customers.

At the origin, I have created the functionality of the deployment directory and the usage of Symbolic Link to try solving a problem for some customers that have difficulties to configure their hosting server to specify the same directory than the master.

To help you test the plesk configuration, I suggest that you use the "Hello World" procedure described in the tutorial step 7