>> Any other logs other than customization logs where i could get additional info?
Sysprep logs are in %WINDIR%\Panther\Unattendgc (especially setupact.log and setuperr.log), there should be an error related to domain join. Could you please post those?
There are also logs in %WINDIR%\Debug, but those usually aren't that useful.
> I have configured DHCP for the VLAN used by external network and the vm is able to obtain ip from DHCP server. The domain name resolves as well.
I would suggest to try to simplify setup even more, e.g. not use VLAN.
Also make sure not to include domain name into user's name:
domain: mydom.com
user: Administrator
pass: <somepass>
OU: <leave empty for testing pursposes, otherwise it should be something like OU=Unit1,DC=mydom,DC=com>
/Andrii
