The manuals do not contain the version itself, but they do tell you which command switch to use to display the version 
On CentOS you can display the installed packages by using yum or rpm, for example:
yum info vzctl
rpm -q vzctl
The vzmigrate script is also a part of the vzctl package and doesn't seem to have a separate version. Perhaps this caused some confusion...
I'm afraid I don't use vzmigrate in the way you do, so I can't help you with the migrate command itself. Someone else will hopefully be able to...