Premium Members unlock the inbox tab in the dashboard which enables them to both see who has left their reviews (as long as the user didn't opt to remain anonymous) as well as respond to reviews.
(Note: On May 1st the free review manager in the vendor dashboard will be discontinued and only premium members will have access to the review manager. Free members will still be able to send individual review requests)
Step 1: Navigate to reviews-->respond to reviews in your vendor dashboard
Step 2a: Click on a review and enter your response
Step 2b: Or you can respond to a review right from your profile
Step 3: Your response will be posted to your profile