Definitely upgrade to 4.3.5, do this first. If you can afford to just image it and start over, do that.
As long as your switch ports are configured correctly, adding vlans is simple. I don't put anything in maintenance to do it.
Just go to networks -> New
Check the "enable VLAN tagging" and enter your vlan. You don't really need to change anything else.
Now you have a logical network and a Vnic profile for this vlan:
Now you need to tell Ovirt what physical NIC you want this to operate on. Go to your host and select "Setup Host Networks"
Drag the new network onto the NIC or bond you want to use:
You can click the pencil and have this interface get an IP address if you want, but, you don't need to - your vms will get IPs. So you can leave this all alone in here:
Now when you are creating a VM, you can attach this Vnic profile. You could also add the Vnic to an existing VM.
And that's it. If you have the VM configured to DHCP, and you have a dhcp server listening on that Vlan, it will work. If your VM doesn't get an IP, check your router's DHCP logs to see if it hears anything from the Mac address of your VM's nic. If you also have a DNS resolver that adds DHCP entries, and your VM has a hostname configured in cloud-init, you'll even be able to resolve the FQDN to your VM immediately.
Hope this helps!