The SAS Visual Investigator Administration guide includes information about custom controls, however, we are working on new documentation to provide more detailed information about how to create custom controls - this new documentation is expected to be available at the time of the next release of SAS Visual Investigator (VI 10.5.1) in late May 2019. Documentation can be accessed via the support pages (see link below) - if you do not have a password for access please contact SAS Technical Support.
http://support.sas.com/documentation/prod-p/visgator/index.html
In terms of getting alert data, you will be restricted to the information that the Alerts REST API exposes as there is no API to read from PostgreSQL tables directly. Please see the product documentation (Admin guide) for more details of the Alerts REST API.