How do I join to and put the computer object in an OU?
You must specify full path to OU.
Here is an example of the command:
Vastool -u Administrator join -c "OU=Unix Users,OU=Users,OU=Domain Users,DC=example,DC=com" example.com
The order of OU's is bottom to top so the OU you want to create the object in will be first, in the above example that would be Unix Users.
Unix Users is in Users and Users is in Domain Users.