If the VMs are a vapp, then you can't use RDM (not supported with vCloud)
You could possibly do NFS shares, and you probably won't use customization.
Example:
You have a vapp with 3 VMs, 2 Linux VMs with Oracle installed and 1 VM that's a Linux VM with NFS server bits installed.
These 3 VMs would be on a vApp network with NAT enabled.
Do all your configurations, capture to catalog and state to make an exact copy.
When you deploy from Catalog to My Cloud, you would deploy it as an exact copy and ensure to retain the NAT setting (you'll get a new external IP)