Easiest way when customer has access to the console of the VM’s:
Create a surebackup job within thi job a Application Group with the DC, the SQL server and ERP system. Set the setting to keep the application group running.
https://helpcenter.veeam.com/docs/backup/vsphere/surebackup_job_appgroup_vm.html?ver=120
In this way, you’ll have an isolated environment that is capable of talking to the DC for authentication etc.
If the customers doens’t have acces, you need to make some additional configuration (Ip masquerading) to make the VM’s available from the production network using natting.
https://helpcenter.veeam.com/docs/backup/vsphere/surebackup_ip_masquerading.html?ver=120