Instantiate-vApp was a cmdlet function written by Clint Kitson. You could download them from his blog at velemental.com and them import all of them to use in your scripts.
Now you can just use the New-CIVApp cmdlet and chose a vApp Template or vApp to clone from.
Are you trying to create vApps from VM's within vSphere? If so let me know as I have a script for this as well. I've done 75 of them this way during the week.
Eric