Hi jkn,
Depending on your situation there are different ways to achieve this.
If you have one Greeting topic and would like to show different messages, but everything else is the same, then you can use a decision (or a scripted text response) and check the value of the context variable called vaContext.portal. It returns the name of the portal, for example "SP" in Service Portal. You can show different messages based on the result.
Please note, that context variables only works instantly when using Service Portal Agent Chat Configuration. If using legacy methods (embedding the Service Portal widget on the portal page), you have to add a sysparm_portal param to the URL.
If you want different setup topics for the different portals (maybe you want complete different experiences) then you should create a different chat experience in Conversational Interfaces > Settings >Virtual Agent > Custom Greetings and setup. Here you can create setup/experience profiles and use different topics for different portal pages. The only thing you need to do is to set a condition in your experience profile that says portal is something.
I hope this helps,
Richard
