Have you tried adding a Magic Date in the VSI Response JSON payload having only a time format? Refer to:
Magic Dates - DevTest Solutions - 10.1 - CA Technologies Documentation
Magic Dates - DevTest Solutions - 10.3 - CA Technologies Documentation
Look for the commentary on: {{=doDateDeltaFromCurrent("yyyy-MM-dd","0D");/2008-12-17}}
To return time only, send the Magic Date processor a valid simple date format with add zero seconds instruction. For example: "JsonTagTimeHHmmss": "{{=doDateDeltaFromCurrent("HH:mm:ss", "0S");/*12:14:31*/}}" or "hh:mm:ss" depending on which format you want
If this is not working, please raise a support ticket.