I just noticed this behavior and it could be my mistake(s) in either construction of the global image or use of it later in the vagrant provisioning.
What I see is that despite explicitly setting sl.hostname and sl.domain to new, unique values, a new instance of a VM that is provided by SoftLayer via the use of sl.image_guid will be provisioned by SL to have the new hostname (good!), a new IP address (whew!), a new DNS record (right!), but the old domain name (bummer!) as it was within the original guest used to form the global image.
I can override that ex post facto, manually and am looking for ways to change it via powershell scripting.
However, I expect that sl.domain = "newDomain.com" should take care of this.
Am I wrong?