For the supervisor to provide their responses, you'll need a VSys Live application. The instructions here assume that you're comfortable with the basics of setting up an application form in VSys Live.
Create a new application form. In its properties, Application type must be VSys Live application, Usage must be External certification update/approval via URL, Certification should be Background 90 day evaluation (or whatever the certification is that you're using). Completed date field, Log field and Posted values field can be set as appropriate.
Don't put any links to this application in VSys Live menus: this application only works when it's being used as part of a reference check.
Place any fields you want onto the application. In this case we're including the volunteer's name, prompting for the supervisor's phone number plus a few fields from the certification (Strengths, Weaknesses, Results and Comments); we added these by dragging them in from under the 90 day evaluation certification heading. The phone number goes only into the application, but the other fields will update the corresponding fields in the certification - that makes them easily reportable.
Save the application.
Never put any private information about the volunteer on this application, or allow editing of fields that you wouldn't want edited by the supervisor. Anyone with access to the link in the generated letters could view and/or edit those fields!
The VSys tech support folks will happily give you a sample template to start with if you want.