Hi!
To get a direct link to the user interaction you can use the getAnswerUrl() and getInteracionUrl() of a workflowToken object. (see the "Send interaction" workflow in library/mail for an example)
Another example (not vCD related, but you get the idea):
You can use the REST API of vCO to answer user interactions programmatically. http://pubs.vmware.com/vsphere-51/topic/com.vmware.vsphere.vco_develop_web_services.doc/GUID-919FED2A-85CB-4EC4-A10E-EDE4CCAD44D5.html
However, you still have to figure out how trigger that from an email.
Another (very user/approver friendly) solution: Create a mailbox for vCO, and let the apporver just reply via email.
In the library you can find example workflows how to check a POP3 mailbox, and with Regular expressions in Javascript it's possible to parse the email content.
Cheers,
Joerg