We can load a standalone Virtual Agent web client onto a browser via URL link: https://instance.service-now.com/$sn-va-web-client-app.do or via iFrame (if embedding into a web page). This web client is by default accessible publicly (even for unauthenticated users) even though the topics are not made public.
The following link explains how to make it make this web client interface non-public & redirects users to login before VA chat
Link - https://support.servicenow.com/kb?id=kb_article_view&sysparm_article=KB0716436
